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/options/options.js

@@ -0,0 +1,48 @@
+const baseUrlInput = document.getElementById('baseUrl');
+const apiKeyInput = document.getElementById('apiKey');
+const statusEl = document.getElementById('status');
+
+async function load() {
+  const { baseUrl, apiKey } = await chrome.storage.local.get(['baseUrl', 'apiKey']);
+  if (baseUrl) baseUrlInput.value = baseUrl;
+  if (apiKey) apiKeyInput.value = apiKey; // deliberately shown back here (options page,
+  // not the popup) so the value can be corrected without regenerating the key.
+}
+
+async function save() {
+  const baseUrl = baseUrlInput.value.trim().replace(/\/$/, '');
+  const apiKey = apiKeyInput.value.trim();
+  await chrome.storage.local.set({ baseUrl, apiKey });
+  statusEl.textContent = 'Saved.';
+  statusEl.style.color = '#16a34a';
+}
+
+async function test() {
+  const { baseUrl, apiKey } = await chrome.storage.local.get(['baseUrl', 'apiKey']);
+  if (!baseUrl || !apiKey) {
+    statusEl.textContent = 'Set both fields and save first.';
+    statusEl.style.color = '#dc2626';
+    return;
+  }
+  statusEl.textContent = 'Testing…';
+  statusEl.style.color = '#64748b';
+  try {
+    const res = await fetch(`${baseUrl}/api/staff/devices`, {
+      headers: { Authorization: `Bearer ${apiKey}` },
+    });
+    if (!res.ok) {
+      const body = await res.json().catch(() => ({}));
+      throw new Error(body.error || `HTTP ${res.status}`);
+    }
+    const devices = await res.json();
+    statusEl.textContent = `Connected — found ${devices.length} tablet(s).`;
+    statusEl.style.color = '#16a34a';
+  } catch (err) {
+    statusEl.textContent = `Failed: ${err.message}`;
+    statusEl.style.color = '#dc2626';
+  }
+}
+
+document.getElementById('save').addEventListener('click', save);
+document.getElementById('test').addEventListener('click', test);
+load();