Add Projects feature; sync Note/Task models with backend additions

## New: Projects
- Project model, API, repository, Riverpod provider
- ProjectListScreen: active/archived sections, create dialog, long-press status/delete
- ProjectSelector widget: DropdownButtonFormField for note + task editors
- Projects tab added to shell (bottom nav + navigation rail, 4th position)
- /projects route registered in GoRouter ShellRoute

## Updated: Note model
- Added tags: List<String>, projectId: int?, milestoneId: int?
- NotesApi.create/update pass tags and project_id
- NotesRepository and NotesNotifier signatures updated
- NoteEditScreen: chip-based tag input + ProjectSelector

## Updated: Task model
- Added projectId: int?, milestoneId: int?, parentId: int?
- TasksApi.create passes project_id; update payload includes project_id
- TasksRepository and TasksNotifier signatures updated
- TaskEditScreen: ProjectSelector added; project_id sent on save

## Provider fix
- ProjectsNotifier.update renamed to updateProject to avoid conflict
  with AsyncNotifier.update(FutureOr<State> Function(State)) base method

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

lib/data/api/tasks_api.dart

@@ -33,6 +33,7 @@ class TasksApi {
     required TaskStatus status,
     required TaskPriority priority,
     DateTime? dueDate,
+    int? projectId,
   }) async {
     try {
       final response = await _dio.post('/api/tasks', data: {
@@ -41,6 +42,7 @@ class TasksApi {
         'status': status.value,
         'priority': priority.value,
         'due_date': dueDate?.toIso8601String(),
+        if (projectId != null) 'project_id': projectId,
       });
       return Task.fromJson(response.data as Map<String, dynamic>);
     } on DioException catch (e) {