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/app.dart

@@ -7,7 +7,9 @@ import 'core/exceptions.dart';
 import 'providers/api_client_provider.dart';
 import 'providers/auth_provider.dart';
 import 'providers/capture_queue_provider.dart';
+import 'providers/milestones_provider.dart';
 import 'providers/notes_provider.dart';
+import 'providers/projects_provider.dart';
 import 'providers/settings_provider.dart';
 import 'providers/update_provider.dart';
 import 'providers/tasks_provider.dart';
@@ -94,7 +96,11 @@ final routerProvider = Provider<GoRouter>((ref) {
       ),
       GoRoute(
         path: Routes.taskNew,
-        builder: (_, _) => const TaskEditScreen(),
+        builder: (_, state) => TaskEditScreen(
+          initialProjectId: state.uri.queryParameters['projectId'] != null
+              ? int.tryParse(state.uri.queryParameters['projectId']!)
+              : null,
+        ),
       ),
       GoRoute(
         path: Routes.taskEdit,
@@ -347,6 +353,8 @@ class _QuickCaptureBarState extends ConsumerState<_QuickCaptureBar> {
         case 'task':
         case 'todo':
           ref.invalidate(tasksProvider);
+          ref.invalidate(projectsProvider);
+          ref.invalidate(projectMilestonesProvider);
       }
 
       final msg = result.message.isNotEmpty
@@ -398,6 +406,8 @@ class _QuickCaptureBarState extends ConsumerState<_QuickCaptureBar> {
           case 'task':
           case 'todo':
             ref.invalidate(tasksProvider);
+            ref.invalidate(projectsProvider);
+            ref.invalidate(projectMilestonesProvider);
         }
       } on NetworkException {
         break; // Still offline — stop draining.