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>

test/shell/app_shell_test.dart

@@ -20,7 +20,6 @@ void main() {
         overrides: [
           // Override the stream provider to return an empty list immediately
           // so that the rooms screen shows the empty state without needing a DB.
-          // ignore: scoped_providers_should_specify_dependencies
           roomWithStatsListProvider.overrideWith(
             (ref) => Stream.value([]),
           ),