feat(app): first-launch prompt to set Open CAD Studio as default for .dwg/.dxf

On the first launch that hasn't answered it yet, the app shows a small
dialog offering to become the default handler for .dwg and .dxf files.
The choice is persisted (default_assoc_prompted) so the prompt never
reappears.

The platform plumbing lives in io/file_association.rs:
  * Windows — opens the OS's own per-app default-programs dialog via
    IApplicationAssociationRegistrationUI::LaunchAdvancedAssociationUI,
    passing the "Open CAD Studio" RegisteredApplications name the MSI
    registers. Modern Windows guards the actual default behind a
    UserChoice hash, so this is the supported consent path.
  * Linux — xdg-mime default against the installed .desktop file, whose
    MimeType entries already cover image/vnd.dwg and image/vnd.dxf.
  * macOS — LSSetDefaultRoleHandlerForContentType binds the DWG/DXF UTIs
    declared in Info.plist to the app bundle id.

The work runs on a dedicated thread (the Windows dialog is modal) with
the result delivered back through a oneshot channel and surfaced in the
command line.

https://claude.ai/code/session_01DFVSKgViFaxEjZD2KsTdtt

src/app/mod.rs

@@ -292,6 +292,12 @@ pub(super) struct OpenCADStudio {
     /// New-release notification window — opened on startup when the
     /// GitHub releases API reports a newer version than this build.
     update_notice_window: Option<window::Id>,
+    /// First-launch "make Open CAD Studio the default for .dwg/.dxf?" prompt
+    /// window. Shown once, gated on `default_assoc_prompted`.
+    assoc_prompt_window: Option<window::Id>,
+    /// Whether the one-time default-association prompt has already been shown.
+    /// Persisted via [`settings::UserSettings`] so it survives restarts.
+    default_assoc_prompted: bool,
     /// Tag of the latest available release (without the leading "v"),
     /// e.g. `"0.3.0"`. `None` when up-to-date or check hasn't returned.
     update_notice_version: Option<String>,
@@ -1045,6 +1051,13 @@ pub enum Message {
     UpdateCheckResult(Option<crate::update_check::UpdateInfo>),
     /// User dismissed the update-notice window.
     UpdateNoticeClose,
+    /// First-launch default-association prompt: user accepted — register this
+    /// app as the default handler for .dwg / .dxf.
+    AssocPromptYes,
+    /// First-launch default-association prompt: user declined (or "not now").
+    AssocPromptNo,
+    /// Result of the platform default-association call.
+    AssocResult(Result<String, String>),
     /// User clicked the "Open release page" button — opens the GitHub
     /// release URL in the OS default browser and closes the notice.
     UpdateNoticeOpenRelease,
@@ -1354,6 +1367,8 @@ impl OpenCADStudio {
             shortcuts_window: None,
             about_window: None,
             update_notice_window: None,
+            assoc_prompt_window: None,
+            default_assoc_prompted: false,
             update_notice_version: None,
             update_notice_body: None,
             clipboard: Vec::new(),
@@ -1581,9 +1596,24 @@ impl OpenCADStudio {
             .filter(|p| !p.as_os_str().to_string_lossy().starts_with('-'))
             .map(|p| Task::done(Message::OpenRecent(p)))
             .unwrap_or_else(Task::none);
+        // One-time prompt offering to make Open CAD Studio the default app for
+        // .dwg / .dxf. Shown only on the first launch that hasn't answered it
+        // yet; the flag is persisted so we never ask twice.
+        let assoc_prompt: Task<Message> = if s.default_assoc_prompted {
+            Task::none()
+        } else {
+            let (id, open) = window::open(window::Settings {
+                size: iced::Size::new(440.0, 210.0),
+                resizable: false,
+                level: window::Level::AlwaysOnTop,
+                ..Default::default()
+            });
+            s.assoc_prompt_window = Some(id);
+            open.map(|_| Message::Noop)
+        };
         (
             s,
-            Task::batch([open_main, check_update, focus_cmd, cli_open]),
+            Task::batch([open_main, check_update, focus_cmd, cli_open, assoc_prompt]),
         )
     }
 }
@@ -1634,6 +1664,9 @@ pub fn run() -> iced::Result {
         if Some(window_id) == state.update_notice_window {
             return "Update Available".into();
         }
+        if Some(window_id) == state.assoc_prompt_window {
+            return "Set Default Application".into();
+        }
         if Some(window_id) == state.unsaved_dialog_window {
             return "Unsaved Changes".into();
         }

src/app/settings.rs

@@ -70,6 +70,10 @@ pub struct UserSettings {
     pub otrack: bool,
     /// Active snap modes, in `SNAP_ORDER`.
     pub snap_modes: Vec<SnapType>,
+    /// Whether the one-time "make Open CAD Studio the default for .dwg/.dxf?"
+    /// prompt has already been shown. Set once the user answers (either way),
+    /// so we never nag again on subsequent launches.
+    pub default_assoc_prompted: bool,
 }
 
 impl Default for UserSettings {
@@ -91,6 +95,7 @@ impl Default for UserSettings {
                 SnapType::Intersection,
                 SnapType::Nearest,
             ],
+            default_assoc_prompted: false,
         }
     }
 }
@@ -124,6 +129,7 @@ impl UserSettings {
                 "grid" => s.show_grid = val == "1",
                 "osnap" => s.snap_enabled = val == "1",
                 "otrack" => s.otrack = val == "1",
+                "default_assoc_prompted" => s.default_assoc_prompted = val == "1",
                 "snap_modes" => {
                     let modes: Vec<SnapType> =
                         val.split(',').filter_map(|t| snap_from_id(t.trim())).collect();
@@ -149,7 +155,7 @@ impl UserSettings {
             .collect::<Vec<_>>()
             .join(",");
         let body = format!(
-            "dyn={}\northo={}\npolar={}\npolar_increment_deg={}\ngrid={}\nosnap={}\notrack={}\nsnap_modes={}\n",
+            "dyn={}\northo={}\npolar={}\npolar_increment_deg={}\ngrid={}\nosnap={}\notrack={}\ndefault_assoc_prompted={}\nsnap_modes={}\n",
             b(self.dyn_input),
             b(self.ortho),
             b(self.polar),
@@ -157,6 +163,7 @@ impl UserSettings {
             b(self.show_grid),
             b(self.snap_enabled),
             b(self.otrack),
+            b(self.default_assoc_prompted),
             modes,
         );
         let _ = std::fs::write(path, body);

src/app/update.rs

@@ -85,6 +85,7 @@ impl OpenCADStudio {
             snap_enabled: self.snapper.snap_enabled,
             otrack: self.snapper.otrack_enabled,
             snap_modes: super::settings::UserSettings::modes_from(self.snapper.enabled.iter()),
+            default_assoc_prompted: self.default_assoc_prompted,
         }
     }
 
@@ -98,6 +99,7 @@ impl OpenCADStudio {
         self.snapper.snap_enabled = s.snap_enabled;
         self.snapper.otrack_enabled = s.otrack;
         self.snapper.enabled = s.snap_modes.iter().copied().collect();
+        self.default_assoc_prompted = s.default_assoc_prompted;
     }
 
     /// Write preferences to disk only when they differ from the last write,
@@ -110,6 +112,13 @@ impl OpenCADStudio {
         }
     }
 
+    /// Record that the one-time default-association prompt has been answered and
+    /// flush it to disk, so the dialog never reappears on later launches.
+    fn mark_assoc_prompted(&mut self) {
+        self.default_assoc_prompted = true;
+        self.persist_settings_if_changed();
+    }
+
     fn update_inner(&mut self, msg: Message) -> Task<Message> {
         match msg {
             Message::Tick(t) => {
@@ -1613,6 +1622,12 @@ impl OpenCADStudio {
                 if self.update_notice_window == Some(id) {
                     self.update_notice_window = None;
                 }
+                // Closing the default-association prompt via the window chrome
+                // counts as answering it — never nag again.
+                if self.assoc_prompt_window == Some(id) {
+                    self.assoc_prompt_window = None;
+                    self.mark_assoc_prompted();
+                }
                 Task::none()
             }
 
@@ -5647,6 +5662,36 @@ impl OpenCADStudio {
                     Task::none()
                 }
             }
+            Message::AssocPromptYes => {
+                self.mark_assoc_prompted();
+                let close = if let Some(id) = self.assoc_prompt_window.take() {
+                    window::close(id)
+                } else {
+                    Task::none()
+                };
+                let run = Task::perform(
+                    crate::io::file_association::set_default_app(),
+                    Message::AssocResult,
+                );
+                Task::batch([close, run])
+            }
+            Message::AssocPromptNo => {
+                self.mark_assoc_prompted();
+                if let Some(id) = self.assoc_prompt_window.take() {
+                    window::close(id)
+                } else {
+                    Task::none()
+                }
+            }
+            Message::AssocResult(result) => {
+                match result {
+                    Ok(msg) => self.command_line.push_info(&msg),
+                    Err(err) => self
+                        .command_line
+                        .push_error(&format!("Could not set default app: {err}")),
+                }
+                Task::none()
+            }
             Message::PageSetupWidthEdit(s) => {
                 self.page_setup_w = s;
                 Task::none()

src/app/view.rs

@@ -473,6 +473,9 @@ impl OpenCADStudio {
             let body = self.update_notice_body.as_deref().unwrap_or("");
             return crate::ui::update_notice::view_window(latest, body);
         }
+        if Some(window_id) == self.assoc_prompt_window {
+            return default_assoc_dialog_window();
+        }
         if Some(window_id) == self.unsaved_dialog_window {
             let tab_name = match &self.pending_close {
                 Some(super::PendingClose::Tab(idx)) => self
@@ -3461,6 +3464,109 @@ fn unsaved_changes_dialog_window(name: &str) -> Element<'static, Message> {
     .into()
 }
 
+/// First-launch prompt offering to register Open CAD Studio as the default
+/// handler for .dwg / .dxf. "Yes" runs the platform association call; "Not now"
+/// just dismisses. Either answer flips the persisted `default_assoc_prompted`
+/// flag so the dialog never reappears.
+fn default_assoc_dialog_window() -> Element<'static, Message> {
+    const BG: Color = Color {
+        r: 0.18,
+        g: 0.18,
+        b: 0.20,
+        a: 1.0,
+    };
+    const BORDER_COL: Color = Color {
+        r: 0.38,
+        g: 0.38,
+        b: 0.42,
+        a: 1.0,
+    };
+    const TEXT_COL: Color = Color {
+        r: 0.90,
+        g: 0.90,
+        b: 0.90,
+        a: 1.0,
+    };
+    const DIM_COL: Color = Color {
+        r: 0.62,
+        g: 0.62,
+        b: 0.66,
+        a: 1.0,
+    };
+    const BTN_YES: Color = Color {
+        r: 0.20,
+        g: 0.46,
+        b: 0.80,
+        a: 1.0,
+    };
+    const BTN_YHOV: Color = Color {
+        r: 0.26,
+        g: 0.55,
+        b: 0.92,
+        a: 1.0,
+    };
+    const BTN_NO: Color = Color {
+        r: 0.28,
+        g: 0.28,
+        b: 0.30,
+        a: 1.0,
+    };
+    const BTN_NHOV: Color = Color {
+        r: 0.36,
+        g: 0.36,
+        b: 0.40,
+        a: 1.0,
+    };
+
+    let btn = |label: &'static str, msg: Message, base: Color, hov: Color| {
+        button(text(label).size(13).color(TEXT_COL))
+            .on_press(msg)
+            .style(move |_: &Theme, status| button::Style {
+                background: Some(Background::Color(match status {
+                    button::Status::Hovered | button::Status::Pressed => hov,
+                    _ => base,
+                })),
+                text_color: TEXT_COL,
+                border: Border {
+                    color: BORDER_COL,
+                    width: 1.0,
+                    radius: 4.0.into(),
+                },
+                shadow: iced::Shadow::default(),
+                snap: false,
+            })
+            .padding([6, 18])
+    };
+
+    container(
+        column![
+            text("Make Open CAD Studio your default CAD app?")
+                .size(15)
+                .color(TEXT_COL),
+            iced::widget::Space::new().height(10),
+            text("Open .dwg and .dxf drawings in Open CAD Studio by default. You can change this later in your system settings.")
+                .size(12)
+                .color(DIM_COL),
+            iced::widget::Space::new().height(22),
+            row![
+                iced::widget::Space::new().width(Fill),
+                btn("Not now", Message::AssocPromptNo, BTN_NO, BTN_NHOV),
+                iced::widget::Space::new().width(8),
+                btn("Yes, set as default", Message::AssocPromptYes, BTN_YES, BTN_YHOV),
+            ]
+            .align_y(iced::Center),
+        ]
+        .spacing(0),
+    )
+    .style(move |_: &Theme| container::Style {
+        background: Some(Background::Color(BG)),
+        ..Default::default()
+    })
+    .center(Fill)
+    .padding([24, 28])
+    .into()
+}
+
 // ── Start / Welcome page ──────────────────────────────────────────────────
 //
 // Renders in place of the model-space viewport when the active tab is the