feat(io): delete .sv$ autosave files on clean save/exit (#205)

The .sv$ recovery copies only need to survive a crash, so they are removed on
a clean save (Save and Save-As both delete the file's .sv$ after writing) and
on a clean exit — a new exit_app() runs cleanup_autosaves over every open tab
before quitting, and all iced::exit() paths now funnel through it. A leftover
.sv$ therefore signals an unclean shutdown, i.e. something to recover.

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

src/app/update/mod.rs

@@ -809,7 +809,7 @@ impl OpenCADStudio {
                         self.pending_close = Some(super::PendingClose::Quit);
                         return self.open_unsaved_dialog_window();
                     }
-                    return iced::exit();
+                    return self.exit_app();
                 }
                 Task::none()
             }
@@ -818,7 +818,7 @@ impl OpenCADStudio {
                 // Only the main window exists now; all dialogs are in-canvas
                 // modals (Plan B). Closing it exits.
                 if self.main_window == Some(id) {
-                    return iced::exit();
+                    return self.exit_app();
                 }
                 Task::none()
             }