feat(06-01): add watchCompletionsForTask DAO method and history localization strings

- Add watchCompletionsForTask(taskId) to TasksDao: Stream<List<TaskCompletion>> sorted newest first
- Regenerate tasks_dao.g.dart with build_runner
- Add taskHistoryTitle, taskHistoryEmpty, taskHistoryCount to app_de.arb
- Regenerate app_localizations.dart and app_localizations_de.dart
- All 5 new DAO tests pass, zero analyze issues

lib/l10n/app_de.arb

@@ -107,5 +107,13 @@
       "overdue": { "type": "int" }
     }
   },
-  "calendarTodayButton": "Heute"
+  "calendarTodayButton": "Heute",
+  "taskHistoryTitle": "Verlauf",
+  "taskHistoryEmpty": "Noch nie erledigt",
+  "taskHistoryCount": "{count} Mal erledigt",
+  "@taskHistoryCount": {
+    "placeholders": {
+      "count": { "type": "int" }
+    }
+  }
 }