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.dart

@@ -273,6 +273,126 @@ abstract class AppLocalizations {
   /// In de, this message translates to:
   /// **'Abbrechen'**
   String get cancel;
+
+  /// No description provided for @taskFormCreateTitle.
+  ///
+  /// In de, this message translates to:
+  /// **'Aufgabe erstellen'**
+  String get taskFormCreateTitle;
+
+  /// No description provided for @taskFormEditTitle.
+  ///
+  /// In de, this message translates to:
+  /// **'Aufgabe bearbeiten'**
+  String get taskFormEditTitle;
+
+  /// No description provided for @taskFormNameLabel.
+  ///
+  /// In de, this message translates to:
+  /// **'Aufgabenname'**
+  String get taskFormNameLabel;
+
+  /// No description provided for @taskFormNameHint.
+  ///
+  /// In de, this message translates to:
+  /// **'z.B. Staubsaugen, Fenster putzen...'**
+  String get taskFormNameHint;
+
+  /// No description provided for @taskFormNameRequired.
+  ///
+  /// In de, this message translates to:
+  /// **'Bitte einen Namen eingeben'**
+  String get taskFormNameRequired;
+
+  /// No description provided for @taskFormFrequencyLabel.
+  ///
+  /// In de, this message translates to:
+  /// **'Wiederholung'**
+  String get taskFormFrequencyLabel;
+
+  /// No description provided for @taskFormFrequencyCustom.
+  ///
+  /// In de, this message translates to:
+  /// **'Benutzerdefiniert'**
+  String get taskFormFrequencyCustom;
+
+  /// No description provided for @taskFormFrequencyEvery.
+  ///
+  /// In de, this message translates to:
+  /// **'Alle'**
+  String get taskFormFrequencyEvery;
+
+  /// No description provided for @taskFormFrequencyUnitDays.
+  ///
+  /// In de, this message translates to:
+  /// **'Tage'**
+  String get taskFormFrequencyUnitDays;
+
+  /// No description provided for @taskFormFrequencyUnitWeeks.
+  ///
+  /// In de, this message translates to:
+  /// **'Wochen'**
+  String get taskFormFrequencyUnitWeeks;
+
+  /// No description provided for @taskFormFrequencyUnitMonths.
+  ///
+  /// In de, this message translates to:
+  /// **'Monate'**
+  String get taskFormFrequencyUnitMonths;
+
+  /// No description provided for @taskFormEffortLabel.
+  ///
+  /// In de, this message translates to:
+  /// **'Aufwand'**
+  String get taskFormEffortLabel;
+
+  /// No description provided for @taskFormDescriptionLabel.
+  ///
+  /// In de, this message translates to:
+  /// **'Beschreibung (optional)'**
+  String get taskFormDescriptionLabel;
+
+  /// No description provided for @taskFormDueDateLabel.
+  ///
+  /// In de, this message translates to:
+  /// **'Erstes Fälligkeitsdatum'**
+  String get taskFormDueDateLabel;
+
+  /// No description provided for @taskDeleteConfirmTitle.
+  ///
+  /// In de, this message translates to:
+  /// **'Aufgabe löschen?'**
+  String get taskDeleteConfirmTitle;
+
+  /// No description provided for @taskDeleteConfirmMessage.
+  ///
+  /// In de, this message translates to:
+  /// **'Die Aufgabe wird unwiderruflich gelöscht.'**
+  String get taskDeleteConfirmMessage;
+
+  /// No description provided for @taskDeleteConfirmAction.
+  ///
+  /// In de, this message translates to:
+  /// **'Löschen'**
+  String get taskDeleteConfirmAction;
+
+  /// No description provided for @taskEmptyTitle.
+  ///
+  /// In de, this message translates to:
+  /// **'Noch keine Aufgaben'**
+  String get taskEmptyTitle;
+
+  /// No description provided for @taskEmptyMessage.
+  ///
+  /// In de, this message translates to:
+  /// **'Erstelle die erste Aufgabe für diesen Raum.'**
+  String get taskEmptyMessage;
+
+  /// No description provided for @taskEmptyAction.
+  ///
+  /// In de, this message translates to:
+  /// **'Aufgabe erstellen'**
+  String get taskEmptyAction;
 }
 
 class _AppLocalizationsDelegate