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/task.dart

@@ -52,6 +52,9 @@ class Task {
   final TaskStatus status;
   final TaskPriority priority;
   final DateTime? dueDate;
+  final int? projectId;
+  final int? milestoneId;
+  final int? parentId;
   final DateTime createdAt;
   final DateTime updatedAt;
 
@@ -62,6 +65,9 @@ class Task {
     required this.status,
     required this.priority,
     this.dueDate,
+    this.projectId,
+    this.milestoneId,
+    this.parentId,
     required this.createdAt,
     required this.updatedAt,
   });
@@ -75,6 +81,9 @@ class Task {
         dueDate: json['due_date'] != null
             ? DateTime.parse(json['due_date'] as String)
             : null,
+        projectId: json['project_id'] as int?,
+        milestoneId: json['milestone_id'] as int?,
+        parentId: json['parent_id'] as int?,
         createdAt: DateTime.parse(json['created_at'] as String),
         updatedAt: DateTime.parse(json['updated_at'] as String),
       );
@@ -85,6 +94,9 @@ class Task {
         'status': status.value,
         'priority': priority.value,
         'due_date': dueDate?.toIso8601String(),
+        'project_id': projectId,
+        'milestone_id': milestoneId,
+        'parent_id': parentId,
       };
 
   Task copyWith({
@@ -93,6 +105,9 @@ class Task {
     TaskStatus? status,
     TaskPriority? priority,
     DateTime? dueDate,
+    Object? projectId = _undefined,
+    Object? milestoneId = _undefined,
+    Object? parentId = _undefined,
   }) =>
       Task(
         id: id,
@@ -101,7 +116,18 @@ class Task {
         status: status ?? this.status,
         priority: priority ?? this.priority,
         dueDate: dueDate ?? this.dueDate,
+        projectId: identical(projectId, _undefined)
+            ? this.projectId
+            : projectId as int?,
+        milestoneId: identical(milestoneId, _undefined)
+            ? this.milestoneId
+            : milestoneId as int?,
+        parentId: identical(parentId, _undefined)
+            ? this.parentId
+            : parentId as int?,
         createdAt: createdAt,
         updatedAt: updatedAt,
       );
+
+  static const _undefined = Object();
 }