feat(02-03): create task providers, form screen with frequency and effort selectors

- TaskActions AsyncNotifier for create, update, delete, complete task mutations
- tasksInRoomProvider manual StreamProvider.family wrapping TasksDao.watchTasksInRoom
- TaskFormScreen with name, frequency (10 presets + custom), effort (3-way segmented),
  description, and initial due date picker (German DD.MM.YYYY format)
- Custom frequency: number + unit picker (Tage/Wochen/Monate)
- Calendar-anchored intervals auto-set anchorDay from due date
- Edit mode loads existing task and pre-fills all fields
- 19 new German localization keys for task form, delete, and empty state

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

lib/features/tasks/presentation/task_providers.dart

@@ -0,0 +1,65 @@
+import 'package:drift/drift.dart' show Value;
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:riverpod_annotation/riverpod_annotation.dart';
+
+import 'package:household_keeper/core/database/database.dart';
+import 'package:household_keeper/core/providers/database_provider.dart';
+import 'package:household_keeper/features/tasks/domain/effort_level.dart';
+import 'package:household_keeper/features/tasks/domain/frequency.dart';
+
+part 'task_providers.g.dart';
+
+/// Stream provider family for tasks in a specific room, sorted by due date.
+///
+/// Defined manually because riverpod_generator has trouble with drift's
+/// generated [Task] type in family provider return types.
+final tasksInRoomProvider =
+    StreamProvider.family.autoDispose<List<Task>, int>((ref, roomId) {
+  final db = ref.watch(appDatabaseProvider);
+  return db.tasksDao.watchTasksInRoom(roomId);
+});
+
+/// Notifier for task mutations: create, update, delete, complete.
+@riverpod
+class TaskActions extends _$TaskActions {
+  @override
+  FutureOr<void> build() {}
+
+  Future<int> createTask({
+    required int roomId,
+    required String name,
+    String? description,
+    required IntervalType intervalType,
+    required int intervalDays,
+    int? anchorDay,
+    required EffortLevel effortLevel,
+    required DateTime nextDueDate,
+  }) async {
+    final db = ref.read(appDatabaseProvider);
+    return db.tasksDao.insertTask(TasksCompanion.insert(
+      roomId: roomId,
+      name: name,
+      description: Value(description),
+      intervalType: intervalType,
+      intervalDays: Value(intervalDays),
+      anchorDay: Value(anchorDay),
+      effortLevel: effortLevel,
+      nextDueDate: nextDueDate,
+    ));
+  }
+
+  Future<void> updateTask(Task task) async {
+    final db = ref.read(appDatabaseProvider);
+    await db.tasksDao.updateTask(task);
+  }
+
+  Future<void> deleteTask(int taskId) async {
+    final db = ref.read(appDatabaseProvider);
+    await db.tasksDao.deleteTask(taskId);
+  }
+
+  Future<void> completeTask(int taskId) async {
+    final db = ref.read(appDatabaseProvider);
+    await db.tasksDao.completeTask(taskId);
+  }
+}