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 <[email protected]>

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,