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_de.arb

@@ -59,5 +59,14 @@
   "taskDeleteConfirmAction": "L\u00f6schen",
   "taskEmptyTitle": "Noch keine Aufgaben",
   "taskEmptyMessage": "Erstelle die erste Aufgabe f\u00fcr diesen Raum.",
-  "taskEmptyAction": "Aufgabe erstellen"
+  "taskEmptyAction": "Aufgabe erstellen",
+  "templatePickerTitle": "Aufgaben aus Vorlagen hinzuf\u00fcgen?",
+  "templatePickerSkip": "\u00dcberspringen",
+  "templatePickerAdd": "Hinzuf\u00fcgen",
+  "templatePickerSelected": "{count} ausgew\u00e4hlt",
+  "@templatePickerSelected": {
+    "placeholders": {
+      "count": { "type": "int" }
+    }
+  }
 }