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>

public/pages/budget.js

@@ -54,6 +54,7 @@ function addMonths(ym, n) {
 // --------------------------------------------------------
 
 async function loadMonth(month) {
+  try {
     const [entriesRes, summaryRes] = await Promise.all([
       api.get(`/budget?month=${month}`),
       api.get(`/budget/summary?month=${month}`),
@@ -61,6 +62,13 @@ async function loadMonth(month) {
     state.month   = month;
     state.entries = entriesRes.data;
     state.summary = summaryRes.data;
+  } catch (err) {
+    console.error('[Budget] loadMonth Fehler:', err);
+    state.month   = month;
+    state.entries = [];
+    state.summary = { income: 0, expenses: 0, balance: 0, by_category: [] };
+    window.oikos?.showToast('Budget konnte nicht geladen werden.', 'danger');
+  }
 }
 
 // --------------------------------------------------------

public/pages/calendar.js

@@ -124,8 +124,14 @@ function eventsOnDay(dateStr) {
 // --------------------------------------------------------
 
 async function loadRange(from, to) {
+  try {
     const res      = await api.get(`/calendar?from=${from}&to=${to}`);
     state.events   = res.data;
+  } catch (err) {
+    console.error('[Calendar] loadRange Fehler:', err);
+    state.events = [];
+    window.oikos?.showToast('Termine konnten nicht geladen werden.', 'danger');
+  }
   state.rangeFrom = from;
   state.rangeTo   = to;
 }

public/pages/meals.js

@@ -74,9 +74,16 @@ function formatDayDate(dateStr) {
 // --------------------------------------------------------
 
 async function loadWeek(week) {
+  try {
     const res = await api.get(`/meals?week=${week}`);
     state.meals       = res.data;
     state.currentWeek = getMondayOf(week);
+  } catch (err) {
+    console.error('[Meals] loadWeek Fehler:', err);
+    state.meals       = [];
+    state.currentWeek = getMondayOf(week);
+    window.oikos?.showToast('Essensplan konnte nicht geladen werden.', 'danger');
+  }
 }
 
 async function loadLists() {

public/pages/notes.js

@@ -58,8 +58,14 @@ export async function render(container, { user }) {
 
   if (window.lucide) lucide.createIcons();
 
+  try {
     const res  = await api.get('/notes');
     state.notes = res.data;
+  } catch (err) {
+    console.error('[Notes] Laden fehlgeschlagen:', err);
+    state.notes = [];
+    window.oikos?.showToast('Notizen konnten nicht geladen werden.', 'danger');
+  }
   renderGrid();
 
   _container.querySelector('#notes-add-btn').addEventListener('click', () => openModal({ mode: 'create' }));

public/pages/shopping.js

@@ -351,8 +351,14 @@ function updateListCounter(listId, totalDelta, checkedDelta) {
 // --------------------------------------------------------
 
 async function loadLists() {
+  try {
     const data   = await api.get('/shopping');
     state.lists  = data.data ?? [];
+  } catch (err) {
+    console.error('[Shopping] loadLists Fehler:', err);
+    state.lists = [];
+    window.oikos?.showToast('Listen konnten nicht geladen werden.', 'danger');
+  }
 }
 
 async function loadItems(listId) {
@@ -366,9 +372,11 @@ async function switchList(listId, container) {
   renderTabs(container);
   try {
     await loadItems(listId);
-  } catch {
+  } catch (err) {
+    console.error('[Shopping] loadItems Fehler:', err);
     state.items = [];
     state.activeList = state.lists.find((l) => l.id === listId) ?? null;
+    window.oikos?.showToast('Artikel konnten nicht geladen werden.', 'danger');
   }
   renderListContent(container);
   wireListContentEvents(container);

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) => {
+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;