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/vite.config.ts

@@ -0,0 +1,32 @@
+import { defineConfig } from 'vite'
+import react from '@vitejs/plugin-react'
+import { VitePWA } from 'vite-plugin-pwa'
+
+export default defineConfig({
+  base: '/calendar/',
+  plugins: [
+    react(),
+    VitePWA({
+      strategies: 'injectManifest',
+      srcDir: 'src',
+      filename: 'sw.js',
+      registerType: 'autoUpdate',
+      manifest: {
+        name: 'Calendar',
+        short_name: 'Calendar',
+        start_url: '/calendar/',
+        scope: '/',
+        display: 'standalone',
+        theme_color: '#c9a84c',
+        background_color: '#c9a84c',
+        icons: [
+          { src: '/calendar/icons/icon-192.png', sizes: '192x192', type: 'image/png' },
+          { src: '/calendar/icons/icon-512.png', sizes: '512x512', type: 'image/png' },
+        ],
+      },
+      injectManifest: {
+        globPatterns: ['**/*.{js,css,html,ico,png,svg}'],
+      },
+    }),
+  ],
+})