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/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 {