Extension: per-machine default tablet, skip asking unless busy

- Options page gains a 'Default tablet for this machine' picker,
  populated from the device list once base URL + terminal key are
  configured, auto-saved on change (small enough setting not to warrant
  its own Save button).
- Popup pre-selects the default in its device dropdown but still shows
  the full picker — opening the popup is already a deliberate action, so
  staff can still override it there.
- Injected buttons (row + banner) skip the dropdown entirely when a
  default is configured — one click pushes straight to it. Only falls
  back to the full picker if that specific push comes back busy (409),
  with a note explaining why it's asking despite a default being set.
  background.js's pushBooking() now returns the HTTP status alongside
  ok/error so this can be detected reliably rather than string-matching
  the error message.

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

backend/extension-src/options/options.html

@@ -28,6 +28,16 @@
   <button id="test">Test connection</button>
   <p id="status"></p>
 
+  <div id="defaultTabletSection" style="display: none;">
+    <label for="defaultDevice">Default tablet for this machine</label>
+    <select id="defaultDevice"></select>
+    <p class="hint">
+      The popup pre-selects this tablet but still lets you pick a different one. The "Send to Tablet"
+      buttons injected into NewBook itself skip asking entirely and push straight to this tablet —
+      unless it's busy, in which case they fall back to asking which one to use.
+    </p>
+  </div>
+
   <script type="module" src="options.js"></script>
 </body>
 </html>