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/models/project.dart

@@ -0,0 +1,40 @@
+class Project {
+  final int id;
+  final String title;
+  final String? description;
+  final String? goal;
+  final String status; // active | completed | archived
+  final String? color;
+  final DateTime createdAt;
+  final DateTime updatedAt;
+
+  const Project({
+    required this.id,
+    required this.title,
+    this.description,
+    this.goal,
+    required this.status,
+    this.color,
+    required this.createdAt,
+    required this.updatedAt,
+  });
+
+  factory Project.fromJson(Map<String, dynamic> json) => Project(
+        id: json['id'] as int,
+        title: json['title'] as String? ?? '',
+        description: json['description'] as String?,
+        goal: json['goal'] as String?,
+        status: json['status'] as String? ?? 'active',
+        color: json['color'] as String?,
+        createdAt: DateTime.parse(json['created_at'] as String),
+        updatedAt: DateTime.parse(json['updated_at'] as String),
+      );
+
+  Map<String, dynamic> toJson() => {
+        'title': title,
+        'description': description,
+        'goal': goal,
+        'status': status,
+        'color': color,
+      };
+}