feat: upgrade Riverpod 3, go_router 17, google_fonts 8, package_info_plus 9

Migrate StateNotifierProvider/StateNotifier → NotifierProvider/Notifier,
StateProvider → NotifierProvider with simple notifier, FamilyAsyncNotifier
removed in Riverpod 3 (replaced with constructor-arg pattern), and rename
.valueOrNull → .value throughout all providers and screens.

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

lib/providers/tasks_provider.dart

@@ -28,14 +28,14 @@ class TasksNotifier extends AsyncNotifier<List<Task>> {
           dueDate: dueDate,
           projectId: projectId,
         );
-    state = AsyncData([...state.valueOrNull ?? [], task]);
+    state = AsyncData([...state.value ?? [], task]);
     return task;
   }
 
   Future<Task> updateTask(int id, Map<String, dynamic> fields) async {
     final updated = await ref.read(tasksRepositoryProvider).update(id, fields);
     state = AsyncData([
-      for (final t in state.valueOrNull ?? [])
+      for (final t in state.value ?? [])
         if (t.id == id) updated else t,
     ]);
     return updated;
@@ -44,7 +44,7 @@ class TasksNotifier extends AsyncNotifier<List<Task>> {
   Future<void> delete(int id) async {
     await ref.read(tasksRepositoryProvider).delete(id);
     state = AsyncData([
-      for (final t in state.valueOrNull ?? [])
+      for (final t in state.value ?? [])
         if (t.id != id) t,
     ]);
   }