fix(flutter): apply Riverpod 3 NotifierProvider conversion missed in 14d7678

The previous fix-forward intended this change but the Edit didn't
make it into the commit (only the Go side landed). Re-applying the
StateProvider → NotifierProvider conversion so flutter analyze stops
failing on the undefined-function for StateProvider.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/update/client_update_provider.dart

@@ -74,7 +74,18 @@ final clientUpdateProvider =
 /// later release's banner re-appears even if the operator dismissed
 /// the previous one. Not persisted — restart re-shows the banner,
 /// which is acceptable nudging for v1.
-final _dismissedVersionsProvider = StateProvider<Set<String>>((_) => <String>{});
+class _DismissedVersionsNotifier extends Notifier<Set<String>> {
+  @override
+  Set<String> build() => <String>{};
+
+  void add(String version) {
+    state = {...state, version};
+  }
+}
+
+final _dismissedVersionsProvider =
+    NotifierProvider<_DismissedVersionsNotifier, Set<String>>(
+        _DismissedVersionsNotifier.new);
 
 /// True when the update banner should render: an UpdateInfo is
 /// available AND the operator hasn't dismissed this specific version.
@@ -89,9 +100,8 @@ final shouldShowUpdateBannerProvider = Provider<UpdateInfo?>((ref) {
 /// Dismiss controller: marks the given version as dismissed so the
 /// banner stops showing for this session.
 final dismissUpdateProvider = Provider<void Function(String)>((ref) {
-  return (version) {
-    ref.read(_dismissedVersionsProvider.notifier).update((s) => {...s, version});
-  };
+  return (version) =>
+      ref.read(_dismissedVersionsProvider.notifier).add(version);
 });
 
 /// Installer provider — depends on dio so the install download uses