feat(save): native OS dialog for Save As, keep in-app format picker

Replace the custom in-app file-browser save dialog with the native OS
save dialog for choosing the destination path — folder browsing and
overwrite confirmation now come from the OS. A compact in-app modal
still selects the DWG/DXF format and version (a native dialog cannot
carry the version year) and appears only for Save As. Plain Save on an
unsaved drawing defaults to DWG 2018 with no picker. The file-name field
is shown only on web, where there is no native dialog.

Remove the hand-rolled directory listing, folder navigation, and
overwrite modal.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -435,8 +435,6 @@ pub(super) struct OpenCADStudio {
     aec_drop_acknowledged: bool,
     /// Number of unsupported objects shown in the AEC-drop warning modal.
     aec_drop_count: usize,
-    /// Set once the user confirms overwriting an existing file on Save-As.
-    overwrite_acknowledged: bool,
     /// Layers awaiting a "delete non-empty layer(s)" confirmation: `(names,
     /// total object count)`. Set when the user deletes one or more layers that
     /// still have objects; the warning modal reads it, and confirming erases
@@ -677,12 +675,9 @@ pub(super) struct OpenCADStudio {
     /// OS window for the custom Save As dialog.
     /// Currently selected format string, e.g. "DWG 2013".
     save_dialog_format: String,
-    /// Editable filename (without path), e.g. "drawing.dwg".
+    /// Editable filename (without path), e.g. "drawing.dwg" — seeds the native
+    /// OS save dialog's default name (native) or the download name (web).
     save_dialog_filename: String,
-    /// Currently browsed folder (PathBuf for reliable fs ops).
-    save_dialog_folder: std::path::PathBuf,
-    /// Cached directory listing: (display_name, is_dir, full_path).
-    save_dialog_entries: Vec<(String, bool, std::path::PathBuf)>,
     /// True when triggered from the unsaved-changes flow.
     save_dialog_for_unsaved: bool,
 
@@ -1097,7 +1092,6 @@ pub enum ModalKind {
     Unsaved,
     SaveDialog,
     AecDropWarning,
-    OverwriteWarning,
     #[cfg_attr(target_arch = "wasm32", allow(dead_code))]
     AssocPrompt,
     PointStyle,
@@ -1234,12 +1228,10 @@ pub enum Message {
     // ── Custom Save-As dialog ─────────────────────────────────────────────
     SaveDialogFormatChanged(String),
     SaveDialogFilenameChanged(String),
-    /// Navigate the file-chooser to the given directory.
-    SaveDialogNavigate(std::path::PathBuf),
-    /// User clicked a file entry (fill filename) or directory entry (navigate).
-    SaveDialogEntryClicked(std::path::PathBuf, bool),
     SaveDialogConfirm,
     SaveDialogCancel,
+    /// Destination picked in the native OS save dialog (`None` = cancelled).
+    SaveDialogPathPicked(Option<std::path::PathBuf>),
     ClearScene,
     SetWireframe(bool),
     /// Set the active tab's render mode (one of acadrust's seven visual
@@ -1285,11 +1277,6 @@ pub enum Message {
     AecDropProceed,
     /// Go back to the Save dialog from the AEC-drop warning.
     AecDropBack,
-    // ── Overwrite confirmation (Save-As over an existing file) ────────────
-    /// Replace the existing file and save.
-    OverwriteConfirm,
-    /// Go back to the Save dialog from the overwrite warning.
-    OverwriteCancel,
     /// Periodic autosave tick — write `.sv$` recovery files for dirty tabs.
     AutoSave,
     /// Save-as path picked for the unsaved-changes → save → close flow.
@@ -2170,7 +2157,6 @@ impl OpenCADStudio {
             active_modal: None,
             aec_drop_acknowledged: false,
             aec_drop_count: 0,
-            overwrite_acknowledged: false,
             layer_delete_pending: None,
             modal_offset: iced::Vector::ZERO,
             modal_drag_last: None,
@@ -2218,11 +2204,6 @@ impl OpenCADStudio {
             pending_close: None,
             save_dialog_format: "DWG 2018".to_string(),
             save_dialog_filename: "drawing.dwg".to_string(),
-            save_dialog_folder: std::env::var("HOME")
-                .or_else(|_| std::env::var("USERPROFILE"))
-                .unwrap_or_else(|_| ".".to_string())
-                .into(),
-            save_dialog_entries: Vec::new(),
             save_dialog_for_unsaved: false,
             // Plot style
             active_plot_style: None,