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/calendar.js

@@ -124,8 +124,14 @@ function eventsOnDay(dateStr) {
 // --------------------------------------------------------
 
 async function loadRange(from, to) {
-  const res      = await api.get(`/calendar?from=${from}&to=${to}`);
-  state.events   = res.data;
+  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;
 }