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/commands/fileops.rs

@@ -211,7 +211,7 @@ impl OpenCADStudio {
                 if let Some(id) = self.main_window {
                     return Some(Task::done(Message::WindowCloseRequested(id)));
                 }
-                return Some(iced::exit());
+                return Some(self.exit_app());
             }
 
             // ── Frame-budget HUD (Phase 5.3) ───────────────────────────────