Fix update dialog looping after download completes

After OpenFile.open() the state was reset to UpdateStatus.available,
which re-triggered the app.dart listener (downloading → available
satisfies the prev != available condition) and showed the dialog again.
Fix: reset to idle so the system installer can proceed uninterrupted.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/providers/update_provider.dart

@@ -124,11 +124,9 @@ class UpdateNotifier extends Notifier<UpdateState> {
         type: 'application/vnd.android.package-archive',
       );
 
-      // Return to available so the user can retry install if they dismissed it.
-      state = state.copyWith(
-        status: UpdateStatus.available,
-        downloadProgress: 1.0,
-      );
+      // Transition to idle — the system installer is now open.
+      // Going back to `available` would re-trigger the update dialog loop.
+      state = const UpdateState();
     } catch (e) {
       state = state.copyWith(
         status: UpdateStatus.error,