Improve project-task integration and sub-task support

- TasksApi: add getSubTasks(parentId) + parentId param to create()
- TasksRepository: expose new API methods
- app.dart: invalidate projectsProvider + projectMilestonesProvider
  after quick capture creates a task; taskNew route reads ?projectId=
  query param and passes as initialProjectId to TaskEditScreen
- project_list_screen.dart: _AddTaskRow passes projectId query param
  so the task editor pre-fills the project on navigation
- task_edit_screen.dart: fully rewritten with initialProjectId support,
  sub-task fetching/creation, and _SubTasksSection widget

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

lib/data/api/tasks_api.dart

@@ -34,6 +34,7 @@ class TasksApi {
     required TaskPriority priority,
     DateTime? dueDate,
     int? projectId,
+    int? parentId,
   }) async {
     try {
       final response = await _dio.post('/api/tasks', data: {
@@ -43,6 +44,7 @@ class TasksApi {
         'priority': priority.value,
         'due_date': dueDate?.toIso8601String(),
         if (projectId != null) 'project_id': projectId,
+        if (parentId != null) 'parent_id': parentId,
       });
       return Task.fromJson(response.data as Map<String, dynamic>);
     } on DioException catch (e) {
@@ -50,6 +52,20 @@ class TasksApi {
     }
   }
 
+  Future<List<Task>> getSubTasks(int parentId) async {
+    try {
+      final response = await _dio.get(
+        '/api/notes',
+        queryParameters: {'parent_id': parentId, 'is_task': 'true', 'limit': 100},
+      );
+      final data = response.data as Map<String, dynamic>;
+      final list = data['notes'] as List<dynamic>;
+      return list.map((e) => Task.fromJson(e as Map<String, dynamic>)).toList();
+    } on DioException catch (e) {
+      throw dioToApp(e);
+    }
+  }
+
   Future<Task> update(int id, Map<String, dynamic> fields) async {
     try {
       final response = await _dio.put('/api/tasks/$id', data: fields);