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-lock.json

@@ -8,6 +8,11 @@
       "name": "fabledassistant-frontend",
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+        "@fullcalendar/daygrid": "^6.1.20",
+        "@fullcalendar/interaction": "^6.1.20",
+        "@fullcalendar/timegrid": "^6.1.20",
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         "@tiptap/core": "^3.0.0",
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         "@tiptap/extension-list": "^3.0.0",
@@ -564,6 +569,55 @@
       "license": "MIT",
       "optional": true
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+      "license": "MIT",
+      "dependencies": {
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+      "peerDependencies": {
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+      "license": "MIT",
+      "peerDependencies": {
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+    "node_modules/@fullcalendar/timegrid": {
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+      "license": "MIT",
+      "dependencies": {
+        "@fullcalendar/daygrid": "~6.1.20"
+      },
+      "peerDependencies": {
+        "@fullcalendar/core": "~6.1.20"
+      }
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+    "node_modules/@fullcalendar/vue3": {
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+      "resolved": "https://registry.npmjs.org/@fullcalendar/vue3/-/vue3-6.1.20.tgz",
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+      "license": "MIT",
+      "peerDependencies": {
+        "@fullcalendar/core": "~6.1.20",
+        "vue": "^3.0.11"
+      }
+    },
     "node_modules/@jridgewell/gen-mapping": {
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       "resolved": "https://registry.npmjs.org/@jridgewell/gen-mapping/-/gen-mapping-0.3.13.tgz",
@@ -2994,6 +3048,16 @@
         "node": "^10 || ^12 || >=14"
       }
     },
+    "node_modules/preact": {
+      "version": "10.12.1",
+      "resolved": "https://registry.npmjs.org/preact/-/preact-10.12.1.tgz",
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+      "license": "MIT",
+      "funding": {
+        "type": "opencollective",
+        "url": "https://opencollective.com/preact"
+      }
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