feat(02-04): create template picker bottom sheet with German localization

- Add TemplatePickerSheet StatefulWidget with checklist UI for task templates
- Add showTemplatePickerSheet helper for modal display with DraggableScrollableSheet
- Add 4 German localization keys: templatePickerTitle, Skip, Add, Selected count
- All templates unchecked by default, confirm button only enabled with selection

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

lib/l10n/app_localizations_de.dart

@@ -164,4 +164,18 @@ class AppLocalizationsDe extends AppLocalizations {
 
   @override
   String get taskEmptyAction => 'Aufgabe erstellen';
+
+  @override
+  String get templatePickerTitle => 'Aufgaben aus Vorlagen hinzuf\u00fcgen?';
+
+  @override
+  String get templatePickerSkip => '\u00dcberspringen';
+
+  @override
+  String get templatePickerAdd => 'Hinzuf\u00fcgen';
+
+  @override
+  String templatePickerSelected(int count) {
+    return '$count ausgew\u00e4hlt';
+  }
 }