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/App.vue

@@ -82,6 +82,7 @@ function onGlobalKeydown(e: KeyboardEvent) {
       case "p": router.push("/projects"); break;
       case "c": router.push("/chat"); break;
       case "g": router.push("/graph"); break;
+      case "l": router.push("/calendar"); break;
     }
     return;
   }
@@ -202,6 +203,12 @@ onUnmounted(() => {
                 <kbd class="shortcut-key">c</kbd>
                 <span class="shortcut-desc">Chat</span>
               </div>
+              <div class="shortcut-row">
+                <kbd class="shortcut-key">g</kbd>
+                <span class="shortcut-key-sep">+</span>
+                <kbd class="shortcut-key">l</kbd>
+                <span class="shortcut-desc">Calendar</span>
+              </div>
               <div class="shortcut-row">
                 <kbd class="shortcut-key">Esc</kbd>
                 <span class="shortcut-desc">Unfocus field → go home</span>