Fix NewBook write mechanism, bundle push extension, remove vestigial hotel_id

- setBookingCustomFields: use bookings_update's own custom_fields array
  (confirmed from the real legacy plugin code) instead of
  instance_custom_fields_set, which was only ever proven for a different,
  unrelated plugin
- online_checkin_complete is now a configurable per-hotel custom field
  (online_checkin_complete_custom_field_id setting), not a bogus top-level
  bookings_update field NewBook would silently ignore
- Fix field-name mismatches vs a real live NewBook booking response:
  booking_arrival/departure/length, guest address (street/city/state/
  postcode/country, not a single field), phone/mobile (in contact_details,
  not top-level), and pickPrimaryGuest now checks primary_client (was
  checking fields that don't exist in the real API)
- Bundle the standalone push extension into backend/extension-src/, add
  GET /api/extension/download (zips on request via archiver) and a
  download button on the Settings page
- Remove newbook_hotel_id — vestigial from the legacy system's multi-hotel
  routing model, never actually reached NewBook's API, this stack is
  single-hotel-per-deployment so there's nothing to route between

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

backend/package.json

@@ -9,6 +9,7 @@
   "dependencies": {
     "@fastify/cookie": "^9.4.0",
     "@fastify/cors": "^9.0.1",
+    "archiver": "^7.0.1",
     "bcryptjs": "^2.4.3",
     "fastify": "^4.28.1",
     "jose": "^5.9.6",