Make the app reachable and quittable after the panel closes

Closing the panel only hid it, and launching the app again did nothing
because the running instance ignored the reopen, so it looked like the
app refused to exit. Reopening now shows the panel, and the main menu
makes Cmd+Q and Cmd+W work while the panel has focus. Termination also
tears down the event tap, hotkeys and sync timers instead of leaving
them live.

Sources/Prompt/SyncService.swift

@@ -67,6 +67,14 @@ final class SyncService: ObservableObject {
         restartPolling()
     }
 
+    func stop() {
+        pollTimer?.invalidate()
+        pollTimer = nil
+        pushWork?.cancel()
+        pushWork = nil
+        Store.shared.onDidChange = nil
+    }
+
     // MARK: - Group membership
 
     func generateCode() {