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/l10n/app_localizations_de.dart

@@ -103,4 +103,65 @@ class AppLocalizationsDe extends AppLocalizations {
 
   @override
   String get cancel => 'Abbrechen';
+
+  @override
+  String get taskFormCreateTitle => 'Aufgabe erstellen';
+
+  @override
+  String get taskFormEditTitle => 'Aufgabe bearbeiten';
+
+  @override
+  String get taskFormNameLabel => 'Aufgabenname';
+
+  @override
+  String get taskFormNameHint => 'z.B. Staubsaugen, Fenster putzen...';
+
+  @override
+  String get taskFormNameRequired => 'Bitte einen Namen eingeben';
+
+  @override
+  String get taskFormFrequencyLabel => 'Wiederholung';
+
+  @override
+  String get taskFormFrequencyCustom => 'Benutzerdefiniert';
+
+  @override
+  String get taskFormFrequencyEvery => 'Alle';
+
+  @override
+  String get taskFormFrequencyUnitDays => 'Tage';
+
+  @override
+  String get taskFormFrequencyUnitWeeks => 'Wochen';
+
+  @override
+  String get taskFormFrequencyUnitMonths => 'Monate';
+
+  @override
+  String get taskFormEffortLabel => 'Aufwand';
+
+  @override
+  String get taskFormDescriptionLabel => 'Beschreibung (optional)';
+
+  @override
+  String get taskFormDueDateLabel => 'Erstes Fälligkeitsdatum';
+
+  @override
+  String get taskDeleteConfirmTitle => 'Aufgabe löschen?';
+
+  @override
+  String get taskDeleteConfirmMessage =>
+      'Die Aufgabe wird unwiderruflich gelöscht.';
+
+  @override
+  String get taskDeleteConfirmAction => 'Löschen';
+
+  @override
+  String get taskEmptyTitle => 'Noch keine Aufgaben';
+
+  @override
+  String get taskEmptyMessage => 'Erstelle die erste Aufgabe für diesen Raum.';
+
+  @override
+  String get taskEmptyAction => 'Aufgabe erstellen';
 }