Fix update dialog: show errors, handle OpenFile result correctly

- Check OpenResult.type before resetting state — previously a failed
  open_file call (e.g. installer blocked on emulator) silently called
  state = const UpdateState(), nulling latestVersion and downloadUrl,
  causing "Version null" and a non-functional Download button
- Show errorMessage in dialog with error colour so failures are visible
- Guard Download button on downloadUrl != null (no button if URL lost)

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

lib/providers/update_provider.dart

@@ -119,14 +119,20 @@ class UpdateNotifier extends Notifier<UpdateState> {
         },
       );
 
-      await OpenFile.open(
+      final result = await OpenFile.open(
         path,
         type: 'application/vnd.android.package-archive',
       );
 
-      // Transition to idle — the system installer is now open.
-      // Going back to `available` would re-trigger the update dialog loop.
-      state = const UpdateState();
+      if (result.type == ResultType.done) {
+        // Installer launched — reset to idle so the dialog closes naturally.
+        state = const UpdateState();
+      } else {
+        state = state.copyWith(
+          status: UpdateStatus.error,
+          errorMessage: 'Could not open installer: ${result.message}',
+        );
+      }
     } catch (e) {
       state = state.copyWith(
         status: UpdateStatus.error,