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/extension-src/popup/popup.html

@@ -0,0 +1,20 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <meta charset="utf-8" />
+  <title>Send to Check-In Tablet</title>
+  <link rel="stylesheet" href="popup.css" />
+</head>
+<body>
+  <div class="header">
+    <span class="title">Check-In Tablet</span>
+    <span id="nbStatus" class="nb-status"></span>
+  </div>
+
+  <div id="content">
+    <p class="muted">Loading…</p>
+  </div>
+
+  <script type="module" src="popup.js"></script>
+</body>
+</html>