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/components/dashboard/widget-registry.tsx

@@ -213,6 +213,16 @@ export function wireSearchWidget(
   }
 }
 
+let calendarWired = false;
+export function wireCalendarWidget(
+  component: ComponentType<WidgetProps>,
+) {
+  if (!calendarWired) {
+    WIDGET_REGISTRY.calendar.component = component;
+    calendarWired = true;
+  }
+}
+
 let noteWired = false;
 export function wireNoteWidget(
   component: ComponentType<WidgetProps>,