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/package.json

@@ -9,6 +9,11 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "@fullcalendar/core": "^6.1.20",
+    "@fullcalendar/daygrid": "^6.1.20",
+    "@fullcalendar/interaction": "^6.1.20",
+    "@fullcalendar/timegrid": "^6.1.20",
+    "@fullcalendar/vue3": "^6.1.20",
     "@tiptap/core": "^3.0.0",
     "@tiptap/extension-link": "^3.0.0",
     "@tiptap/extension-list": "^3.0.0",