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/notes_provider.dart

@@ -12,16 +12,35 @@ class NotesNotifier extends AsyncNotifier<List<Note>> {
     return ref.watch(notesRepositoryProvider).getAll();
   }
 
-  Future<Note> create(String title, String body) async {
-    final note =
-        await ref.read(notesRepositoryProvider).create(title, body);
+  Future<Note> create(
+    String title,
+    String body, {
+    List<String> tags = const [],
+    int? projectId,
+  }) async {
+    final note = await ref
+        .read(notesRepositoryProvider)
+        .create(title, body, tags: tags, projectId: projectId);
     state = AsyncData([...state.valueOrNull ?? [], note]);
     return note;
   }
 
-  Future<Note> updateNote(int id, String title, String body) async {
-    final updated =
-        await ref.read(notesRepositoryProvider).update(id, title, body);
+  Future<Note> updateNote(
+    int id,
+    String title,
+    String body, {
+    List<String> tags = const [],
+    int? projectId,
+    bool clearProject = false,
+  }) async {
+    final updated = await ref.read(notesRepositoryProvider).update(
+          id,
+          title,
+          body,
+          tags: tags,
+          projectId: projectId,
+          clearProject: clearProject,
+        );
     state = AsyncData([
       for (final n in state.valueOrNull ?? [])
         if (n.id == id) updated else n,