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/en.json

@@ -130,6 +130,7 @@
     "search.placeholder": "Search...",
     "calendar.name": "Calendar",
     "calendar.description": "Upcoming events",
+    "calendar.loading": "Loading...",
     "calendar.emptyNoSources": "No calendar sources configured",
     "calendar.emptyNoEvents": "No upcoming events",
     "calendar.connectionSuccess": "Connection successful",