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/providers/tasks_provider.dart

@@ -18,6 +18,7 @@ class TasksNotifier extends AsyncNotifier<List<Task>> {
     required TaskStatus status,
     required TaskPriority priority,
     DateTime? dueDate,
+    int? projectId,
   }) async {
     final task = await ref.read(tasksRepositoryProvider).create(
           title: title,
@@ -25,6 +26,7 @@ class TasksNotifier extends AsyncNotifier<List<Task>> {
           status: status,
           priority: priority,
           dueDate: dueDate,
+          projectId: projectId,
         );
     state = AsyncData([...state.valueOrNull ?? [], task]);
     return task;