feat(io): .bak backup-on-save + open .bak as a supported type (#205)

- Save: overwriting an existing file first copies it to a sibling <name>.bak
  (default on; toggle with the ISAVEBAK command, persisted in settings).
- Open: .bak is a first-class type — the Open dialog lists it and the loader
  detects DWG vs DXF from the file's leading magic bytes (the extension hides
  the real format).
- File association registers .bak too, every launch (so users who associated
  earlier get it automatically): Windows via the registry; Linux via a
  high-priority shared-mime-info magic that resolves DWG/DXF content under the
  .bak extension, with the generic *.bak glob claimed only once the app is the
  default DWG handler — so it never hijacks other apps' backups.

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

src/app/commands/styleprops.rs

@@ -1449,6 +1449,25 @@ impl OpenCADStudio {
                         .push_error("Requires 0 OR 1 OR MULTIPLE OR SINGLE");
                 }
             }
+            "ISAVEBAK" => {
+                let v = if self.backup_on_save { 1 } else { 0 };
+                self.command_line.push_output(&format!("ISAVEBAK = {v}"));
+            }
+            cmd if cmd.starts_with("ISAVEBAK ") => {
+                match cmd.trim_start_matches("ISAVEBAK").trim() {
+                    "0" => {
+                        self.backup_on_save = false;
+                        self.persist_settings_if_changed();
+                        self.command_line.push_output("ISAVEBAK set to 0");
+                    }
+                    "1" => {
+                        self.backup_on_save = true;
+                        self.persist_settings_if_changed();
+                        self.command_line.push_output("ISAVEBAK set to 1");
+                    }
+                    _ => self.command_line.push_error("Requires 0 or 1"),
+                }
+            }
             "PDSIZE" => {
                 use crate::command::ValuePromptCommand;
                 let c = ValuePromptCommand::new(

src/app/mod.rs

@@ -247,6 +247,9 @@ pub(super) struct OpenCADStudio {
     dyn_input: bool,
     /// Controls whether the TEXTEDIT command repeats automatically (0 = Multiple, 1 = Single).
     pub texteditmode: bool,
+    /// When true (default), saving over an existing file first writes a `.bak`
+    /// copy of it for recovery (#205). Toggle with the ISAVEBAK command.
+    pub backup_on_save: bool,
     /// Persisted default viewport background, restored from settings and applied
     /// to every drawing tab (new and opened) so a chosen background survives
     /// restarts (#188). `None` = the built-in dark-grey / off-white defaults.
@@ -1862,6 +1865,7 @@ impl OpenCADStudio {
             show_grid: false,
             dyn_input: true,
             texteditmode: false,
+            backup_on_save: true,
             default_bg_color: None,
             default_paper_bg_color: None,
             awaiting_vports: false,

src/app/settings.rs

@@ -103,6 +103,9 @@ pub struct UserSettings {
     pub plugin_repos: Vec<String>,
     /// Controls whether the TEXTEDIT command repeats automatically (0 = Multiple, 1 = Single).
     pub texteditmode: bool,
+    /// When true, saving over an existing file first copies it to a sibling
+    /// `<name>.bak` so a faulty or accidental save can be recovered (#205).
+    pub backup_on_save: bool,
     /// Persisted viewport background colours (0–255 RGB); `None` = app default
     /// (dark grey model / off-white paper). Applied to every drawing tab on
     /// launch and to tabs opened later, so a chosen background survives restarts
@@ -133,6 +136,7 @@ impl Default for UserSettings {
             disabled_plugins: Vec::new(),
             plugin_repos: Vec::new(),
             texteditmode: false,
+            backup_on_save: true,
             bg_color: None,
             paper_bg_color: None,
         }
@@ -177,6 +181,7 @@ impl UserSettings {
                         s.texteditmode = v;
                     }
                 }
+                "backup_on_save" => s.backup_on_save = val == "1",
                 "disabled_plugins" => {
                     s.disabled_plugins = val
                         .split(',')
@@ -218,7 +223,7 @@ impl UserSettings {
             .collect::<Vec<_>>()
             .join(",");
         let body = format!(
-            "dyn={}\northo={}\npolar={}\npolar_increment_deg={}\nosnap={}\notrack={}\ndefault_assoc_prompted={}\nsnap_modes={}\ndisabled_plugins={}\nplugin_repos={}\ntexteditmode={}\nbg_color={}\npaper_bg_color={}\n",
+            "dyn={}\northo={}\npolar={}\npolar_increment_deg={}\nosnap={}\notrack={}\ndefault_assoc_prompted={}\nsnap_modes={}\ndisabled_plugins={}\nplugin_repos={}\ntexteditmode={}\nbackup_on_save={}\nbg_color={}\npaper_bg_color={}\n",
             b(self.dyn_input),
             b(self.ortho),
             b(self.polar),
@@ -230,6 +235,7 @@ impl UserSettings {
             self.disabled_plugins.join(","),
             self.plugin_repos.join(","),
             self.texteditmode,
+            b(self.backup_on_save),
             rgb_to_str(self.bg_color),
             rgb_to_str(self.paper_bg_color),
         );

src/app/update/file.rs

@@ -40,6 +40,7 @@ impl OpenCADStudio {
             },
             plugin_repos: self.plugin_repos.clone(),
             texteditmode: self.texteditmode,
+            backup_on_save: self.backup_on_save,
             bg_color: self.default_bg_color.map(f4_to_u3),
             paper_bg_color: self.default_paper_bg_color.map(f4_to_u3),
         }
@@ -58,6 +59,7 @@ impl OpenCADStudio {
         self.disabled_plugins = s.disabled_plugins.iter().cloned().collect();
         self.plugin_repos = s.plugin_repos.clone();
         self.texteditmode = s.texteditmode;
+        self.backup_on_save = s.backup_on_save;
         self.default_bg_color = s.bg_color.map(u3_to_f4);
         self.default_paper_bg_color = s.paper_bg_color.map(u3_to_f4);
         // Push the restored background onto every drawing tab that exists now
@@ -536,6 +538,9 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
                     self.tabs[i].scene.document.header.user_real1 =
                         self.tabs[i].scene.annotation_scale as f64;
                     // A direct Save preserves the document's current version.
+                    if self.backup_on_save {
+                        crate::io::write_backup(&path);
+                    }
                     match crate::io::save(&self.tabs[i].scene.document, &path) {
                         Ok(()) => {
                             self.command_line
@@ -597,6 +602,9 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
                 #[cfg(not(target_arch = "wasm32"))]
                 {
                     let path = self.save_dialog_folder.join(&filename);
+                    if self.backup_on_save {
+                        crate::io::write_backup(&path);
+                    }
                     match crate::io::save_as_version(&self.tabs[i].scene.document, &path, version) {
                         Ok(()) => {
                             self.command_line

src/app/update/mod.rs

@@ -315,9 +315,10 @@ impl OpenCADStudio {
                 async {
                     let handle = rfd::AsyncFileDialog::new()
                         .set_title("Select External Reference File")
-                        .add_filter("CAD Files", &["dwg", "dxf", "DWG", "DXF"])
+                        .add_filter("CAD Files", &["dwg", "dxf", "bak", "DWG", "DXF", "BAK"])
                         .add_filter("DWG Files", &["dwg", "DWG"])
                         .add_filter("DXF Files", &["dxf", "DXF"])
+                        .add_filter("Backup Files", &["bak", "BAK"])
                         .pick_file()
                         .await;
                     match handle {