feat(05-04): calendar API client, calendar widget, registry wiring

- calendar-api.ts: fetchSources/addSource/updateSource/deleteSource/testSource/fetchEvents with credentials:'include'
- calendar-widget.tsx: upcoming-events list with source color dots, three empty states (no sources/no events/loading)
- widget-registry.tsx: wireCalendarWidget() replaces placeholder with real CalendarWidget
- page.tsx: wires CalendarWidget into registry on mount
- i18n: added calendar.loading key to de.json and en.json

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

apps/web/src/messages/de.json

@@ -130,6 +130,7 @@
     "search.placeholder": "Suchen...",
     "calendar.name": "Kalender",
     "calendar.description": "Kommende Termine",
+    "calendar.loading": "Laden...",
     "calendar.emptyNoSources": "Keine Kalenderquellen konfiguriert",
     "calendar.emptyNoEvents": "Keine anstehenden Termine",
     "calendar.connectionSuccess": "Verbindung erfolgreich",