feat(calendar): internal event store with FullCalendar UI and CalDAV push sync

- AI calendar tools now always available (moved from _CALDAV_TOOLS to _CORE_TOOLS);
  create/list/search/update/delete events go through the internal DB store first,
  with fire-and-forget CalDAV push sync when the user has CalDAV configured
- Add EventEntry interface and typed API helpers (listEvents, createEvent,
  getEvent, updateEvent, deleteEvent) to client.ts
- Install @fullcalendar/vue3, daygrid, timegrid, interaction, core packages
- Add EventSlideOver.vue: create/edit/delete slide-over with title, start/end,
  all-day toggle, location, description, color picker, and project selector
- Add CalendarView.vue: month/week/day FullCalendar with drag-drop and resize
  wired to PATCH /api/events/:id; click empty date opens create slide-over
- Wire /calendar route, Calendar nav link in AppHeader, g+l keyboard shortcut

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

frontend/src/router/index.ts

@@ -110,6 +110,11 @@ const router = createRouter({
       name: "shared-with-me",
       component: () => import("@/views/SharedWithMeView.vue"),
     },
+    {
+      path: "/calendar",
+      name: "calendar",
+      component: () => import("@/views/CalendarView.vue"),
+    },
     {
       path: "/briefing",
       name: "briefing",