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.dart

@@ -393,6 +393,30 @@ abstract class AppLocalizations {
   /// In de, this message translates to:
   /// **'Aufgabe erstellen'**
   String get taskEmptyAction;
+
+  /// No description provided for @templatePickerTitle.
+  ///
+  /// In de, this message translates to:
+  /// **'Aufgaben aus Vorlagen hinzuf\u00fcgen?'**
+  String get templatePickerTitle;
+
+  /// No description provided for @templatePickerSkip.
+  ///
+  /// In de, this message translates to:
+  /// **'\u00dcberspringen'**
+  String get templatePickerSkip;
+
+  /// No description provided for @templatePickerAdd.
+  ///
+  /// In de, this message translates to:
+  /// **'Hinzuf\u00fcgen'**
+  String get templatePickerAdd;
+
+  /// No description provided for @templatePickerSelected.
+  ///
+  /// In de, this message translates to:
+  /// **'{count} ausgew\u00e4hlt'**
+  String templatePickerSelected(int count);
 }
 
 class _AppLocalizationsDelegate