feat(02-02): create room providers, form screen, icon picker, and router routes

- Add Riverpod providers (roomWithStatsList, RoomActions) connecting to RoomsDao
- Create RoomFormScreen with name field, icon picker preview, create/edit modes
- Create IconPickerSheet bottom sheet with curated Material Icons grid
- Add nested GoRouter routes: /rooms/new, /rooms/:roomId, /rooms/:roomId/edit
- Add placeholder TaskListScreen and TaskFormScreen for Plan 03 routes
- Add 11 new German localization keys for room management UI
- Add flutter_reorderable_grid_view dependency

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

lib/features/tasks/presentation/task_form_screen.dart

@@ -0,0 +1,18 @@
+import 'package:flutter/material.dart';
+
+/// Placeholder for the task creation/edit form.
+/// Will be fully implemented in Plan 03.
+class TaskFormScreen extends StatelessWidget {
+  const TaskFormScreen({super.key, this.roomId, this.taskId});
+
+  final int? roomId;
+  final int? taskId;
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: const Text('Aufgabe')),
+      body: const Center(child: Text('Demnächst verfügbar')),
+    );
+  }
+}