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/README.md

@@ -0,0 +1,67 @@
+# Send to Check-In Tablet
+
+Standalone Chrome extension — deliberately separate from `chrome-newbook-assistant`
+(see `project_checkin` memory's "Standalone push extension" decision). Its one job:
+detect a NewBook booking page and push that booking to a physical check-in tablet
+running the [`tablet-checkin`](../tablet-checkin) app.
+
+## How it works
+
+- **Detection**: a background service worker watches for NewBook booking pages
+  (`bookings_view/<id>`, `bookings_edit/<id>`, `bookings_checkin/<id>` — confirmed real
+  URLs, 2026-09-01) and shows a badge on the toolbar icon when one's open. The
+  SPA-navigation handling (`webNavigation.onHistoryStateUpdated`) is lifted directly from
+  `chrome-newbook-assistant/background.js` (`~/laptop-archive`) — already proven against
+  real NewBook navigation, not guessed here.
+- **Popup**: click the toolbar icon → shows the detected booking number, a device
+  picker (busy tablets shown disabled), and a "Send to Check-In Tablet" button.
+- **Row-injected button**: on NewBook's "due for arrival" list
+  (`bookings_lists_checkin`), a content script adds a "Send to Tablet" button under each
+  row's own Check In button, with an inline device picker. Row detection and the
+  booking-ID extraction are built from a real sample row the user provided (2026-09-01,
+  see `project_checkin` memory) — deliberately doesn't rely on any `id` attribute from
+  NewBook's own markup, since its Check In button reuses the same `id` across every row
+  (`generic_button_1`), only class/attribute selectors scoped per row. A
+  `MutationObserver` re-scans for new rows since this page refreshes its table via AJAX
+  without a full reload.
+- **NewBook login indicator**: the popup shows whether the browser looks logged into
+  NewBook (cookie-based check, also lifted from `chrome-newbook-assistant`'s
+  `AuthManager.checkNewBookAuth`). This is a **UX signal only** — it does not authorize
+  anything. The actual push call authenticates with a terminal API key (below), not the
+  NewBook cookie.
+- **Auth**: the push call itself is authorized by a **terminal API key**, generated from
+  tablet-checkin's own Terminals admin page (`admin` capability) and pasted into this
+  extension's Settings page. This mirrors the same trust model already used for the
+  tablets themselves — hotel-owned hardware gets a device-level credential, not a
+  per-user login. See `project_checkin` memory's "Extension auth" section for the full
+  reasoning.
+
+## Setup
+
+1. Deploy `tablet-checkin` and note its base URL.
+2. In tablet-checkin, go to **Terminals** (requires `admin` capability) → create a
+   terminal (e.g. "Reception PC 1") → copy the key shown (only shown once).
+3. Load this extension unpacked: `chrome://extensions` → Developer mode → **Load
+   unpacked** → select this folder.
+4. Click the extension icon → **Open Settings** (or right-click the icon → Options) →
+   enter the tablet-checkin base URL and the terminal key → Save → Test connection.
+
+## Known TODOs before this is production-ready
+
+- **Row-detection selectors on `bookings_lists_checkin` are built from a single sample
+  row** (2026-09-01) — robust in principle (class/attribute-based, not relying on
+  NewBook's non-unique ids), but only actually tested against one row's markup so far.
+  Worth confirming against a live page with multiple/varied rows (different booking
+  statuses, group bookings, etc.) before relying on it daily.
+- `host_permissions` in `manifest.json` only covers the NewBook domains (needed for the
+  cookie-detection API) — tablet-checkin's own backend host isn't declared there since
+  its production URL isn't fixed yet. Cross-origin `fetch()` calls from the popup/options
+  pages generally work regardless, but add the real host once known if CORS issues show
+  up.
+- Icons are placeholder-generated (simple crimson square + gold arrow, matching
+  tablet-checkin's theme color) — replace if a proper design is wanted.
+- Not yet published anywhere (Chrome Web Store or otherwise) — currently load-unpacked
+  only, meaning it needs manually installing on every reception PC. tablet-checkin's own
+  Settings page has a download link (a bundled copy of this source, zipped on request) as
+  of 2026-09-01 — keep `backend/extension-src/` in that repo in sync with this folder,
+  they are not currently the same copy.