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
 // --------------------------------------------------------

server/routes/contacts.js

@@ -151,8 +151,13 @@ router.delete('/:id', (req, res) => {
  * Kategorien-Liste für Dropdowns.
  * Response: { data: { categories } }
  */
-router.get('/meta', (req, res) => {
-  res.json({ data: { categories: VALID_CATEGORIES } });
+router.get('/meta', (_req, res) => {
+  try {
+    res.json({ data: { categories: VALID_CATEGORIES } });
+  } catch (err) {
+    console.error('[contacts/GET /meta]', err);
+    res.status(500).json({ error: 'Interner Fehler', code: 500 });
+  }
 });
 
 module.exports = router;

server/routes/dashboard.js

@@ -25,6 +25,7 @@ const db = require('../db');
  * }
  */
 router.get('/', (req, res) => {
+  try {
   const d = db.get();
   const result = {};
 
@@ -116,6 +117,10 @@ router.get('/', (req, res) => {
   }
 
   res.json(result);
+  } catch (err) {
+    console.error('[Dashboard] Kritischer Fehler:', err.message);
+    res.status(500).json({ error: 'Dashboard konnte nicht geladen werden.', code: 500 });
+  }
 });
 
 module.exports = router;