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>

frontend/src/pages/Settings.tsx

@@ -1,14 +1,14 @@
 import { useEffect, useState } from 'react'
-import { MonitorCog } from 'lucide-react'
+import { MonitorCog, Download } from 'lucide-react'
 import {
   fetchSettings, updateSettings, testNewbookConnection,
-  fetchKioskPasswordStatus, setKioskPassword,
+  fetchKioskPasswordStatus, setKioskPassword, extensionDownloadUrl,
 } from '../api'
 import type { AppSettings } from '../types'
 
 export default function Settings() {
   const [settings, setSettings] = useState<AppSettings>({
-    newbook_hotel_id: '', regcard_custom_field_id: '',
+    regcard_custom_field_id: '',
     online_checkin_complete_custom_field_id: '', terms_and_conditions_text: '',
   })
   const [loading, setLoading] = useState(true)
@@ -76,13 +76,6 @@ export default function Settings() {
 
       <div className="section-title">NewBook Integration</div>
       <div className="field-row">
-        <div className="field">
-          <label>NewBook Hotel / Location ID</label>
-          <input
-            type="text" value={settings.newbook_hotel_id}
-            onChange={e => setSettings(s => ({ ...s, newbook_hotel_id: e.target.value }))}
-          />
-        </div>
         <div className="field">
           <label>Regcard PDF Link — Custom Field ID</label>
           <input
@@ -127,6 +120,20 @@ export default function Settings() {
         </button>
       </div>
 
+      <div className="section-title">Push Extension</div>
+      <p className="field-hint" style={{ marginBottom: 10 }}>
+        Standalone Chrome extension — adds a "Send to Check-In Tablet" button when a NewBook booking
+        is open. Not published anywhere yet, so it needs installing manually on each reception PC:
+        download, unzip, then <code>chrome://extensions</code> → enable Developer mode → Load
+        unpacked → select the unzipped folder. It'll need a Terminal API key from the Terminals page
+        to actually authorize pushes — create one there first.
+      </p>
+      <div style={{ marginBottom: 24 }}>
+        <a className="btn" href={extensionDownloadUrl()} style={{ display: 'inline-flex', alignItems: 'center', gap: 6, textDecoration: 'none' }}>
+          <Download size={16} strokeWidth={1.75} /> Download Extension (.zip)
+        </a>
+      </div>
+
       <div className="section-title">Kiosk Tablets</div>
       <p className="field-hint" style={{ marginBottom: 10 }}>
         A shared password every check-in tablet uses to set itself up at <code>/tablet-checkin/kiosk</code> —