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/repositories/tasks_repository.dart

@@ -15,6 +15,7 @@ class TasksRepository {
     required TaskPriority priority,
     DateTime? dueDate,
     int? projectId,
+    int? parentId,
   }) =>
       _api.create(
         title: title,
@@ -23,8 +24,11 @@ class TasksRepository {
         priority: priority,
         dueDate: dueDate,
         projectId: projectId,
+        parentId: parentId,
       );
 
+  Future<List<Task>> getSubTasks(int parentId) => _api.getSubTasks(parentId);
+
   Future<Task> update(int id, Map<String, dynamic> fields) =>
       _api.update(id, fields);