fix: Error Handling in Backend und Frontend verbessern (Phase 5, Schritt 31)

- Backend: JSON-Parse-Error + Payload-Too-Large Middleware in index.js
- Backend: Dashboard äußerer try/catch für db.get()-Fehler
- Backend: contacts/meta Route mit try/catch
- Frontend: try/catch + Toast-Fallback in loadMonth (budget), loadRange (calendar),
  loadWeek (meals), loadLists/switchList (shopping), initiales Laden (notes)

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

server/index.js

@@ -67,6 +67,17 @@ app.set('trust proxy', 1);
 app.use(express.json({ limit: '1mb' }));
 app.use(express.urlencoded({ extended: true, limit: '1mb' }));
 
+// JSON-Parse-Fehler abfangen (gibt sonst HTML zurück)
+app.use((err, req, res, next) => {
+  if (err.type === 'entity.parse.failed') {
+    return res.status(400).json({ error: 'Ungültiges JSON im Request-Body.', code: 400 });
+  }
+  if (err.type === 'entity.too.large') {
+    return res.status(413).json({ error: 'Request-Body zu groß (max. 1 MB).', code: 413 });
+  }
+  next(err);
+});
+
 // --------------------------------------------------------
 // Sessions
 // --------------------------------------------------------