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.css

@@ -0,0 +1,32 @@
+body {
+  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
+  max-width: 480px;
+  margin: 32px auto;
+  padding: 0 16px;
+  color: #1e293b;
+}
+h1 { font-size: 18px; color: #1a1a2e; }
+label { display: block; margin-top: 16px; font-weight: 600; font-size: 13px; }
+input {
+  width: 100%;
+  box-sizing: border-box;
+  padding: 8px 10px;
+  margin-top: 4px;
+  border: 1px solid #e4e8ee;
+  border-radius: 8px;
+  font-size: 13px;
+}
+.hint { color: #64748b; font-size: 12px; margin: 4px 0 0; }
+button {
+  margin-top: 20px;
+  margin-right: 8px;
+  padding: 8px 16px;
+  border: none;
+  border-radius: 8px;
+  background: #c9a84c;
+  color: #1a1a2e;
+  font-weight: 600;
+  cursor: pointer;
+}
+button#test { background: #f4f5f7; color: #1e293b; border: 1px solid #e4e8ee; }
+#status { margin-top: 12px; font-size: 13px; }

backend/extension-src/options/options.html

@@ -0,0 +1,33 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <meta charset="utf-8" />
+  <title>Send to Check-In Tablet — Settings</title>
+  <link rel="stylesheet" href="options.css" />
+</head>
+<body>
+  <h1>Send to Check-In Tablet</h1>
+  <p class="hint">
+    Two things needed before this extension can push a booking to a tablet — both come from the
+    <strong>tablet-checkin</strong> app itself, not from here.
+  </p>
+
+  <label for="baseUrl">Tablet Check-In base URL</label>
+  <input id="baseUrl" type="text" placeholder="https://<host>/tablet-checkin — not confirmed yet, see TODO in background.js" />
+  <p class="hint">The app's own address once deployed. Include no trailing slash.</p>
+
+  <label for="apiKey">Terminal API key</label>
+  <input id="apiKey" type="password" placeholder="Paste the key shown once when you created this terminal" />
+  <p class="hint">
+    Create one from tablet-checkin's <strong>Terminals</strong> admin page (requires the <code>admin</code>
+    capability). The raw key is shown exactly once at creation — if you've lost it, revoke it there and
+    create a new one.
+  </p>
+
+  <button id="save">Save</button>
+  <button id="test">Test connection</button>
+  <p id="status"></p>
+
+  <script type="module" src="options.js"></script>
+</body>
+</html>

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();