Extension: Send to Tablet on the checkin page, popup auto-push, reset, new icon

- Inject Send to Tablet into the bookings_checkin page's top/bottom context
  menus (left of Confirm Check-In) and a dedicated banner above the tabs
- Popup now auto-pushes to the default tablet instead of just pre-selecting
  it, with a terminal-key-authed reset route so reception can clear a
  stuck-busy tablet from the extension popup
- Replace the extension icon with a person+checkmark glyph matching the
  app's own UserCheck icon and accent colour

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/extension-src/manifest.json

@@ -21,7 +21,8 @@
     {
       "matches": [
         "https://appeu.newbook.cloud/bookings_lists_checkin*",
-        "https://appeu.newbook.cloud/bookings_view/*"
+        "https://appeu.newbook.cloud/bookings_view/*",
+        "https://appeu.newbook.cloud/bookings_checkin/*"
       ],
       "js": ["content-script.js"],
       "run_at": "document_idle"