Add calendar app — shared events calendar with departments, staff tagging, bank holidays and two-way phone sync

Fastify/Postgres backend + React frontend matching the stack's app conventions, plus
a hand-rolled RFC 4791 CalDAV server (caldav-adapter turned out Koa-only in practice)
so calendars subscribe as genuine two-way sync in Apple/Google/Outlook. v1 scope:
multiple colour-coded calendars, department/staff event tagging via live Workforce
lookups, month/week/day/list views, dashboard, file attachments, activity log, and
an auto-synced UK bank holidays calendar. Verified end-to-end locally against real
Postgres: REST CRUD, CalDAV discovery/PROPFIND/REPORT/PUT/sync-collection, all-day
date handling, system-calendar write protection, and activity logging across both
the web and CalDAV write paths.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/tsconfig.json

@@ -0,0 +1,19 @@
+{
+  "compilerOptions": {
+    "target": "ES2020",
+    "useDefineForClassFields": true,
+    "lib": ["ES2020", "DOM", "DOM.Iterable"],
+    "module": "ESNext",
+    "skipLibCheck": true,
+    "moduleResolution": "bundler",
+    "allowImportingTsExtensions": true,
+    "resolveJsonModule": true,
+    "isolatedModules": true,
+    "noEmit": true,
+    "jsx": "react-jsx",
+    "strict": true,
+    "noUnusedLocals": false,
+    "noUnusedParameters": false
+  },
+  "include": ["src"]
+}