Extension: fix duplicate checkin banner and silently-unsaved default tablet

- Scope the new bookings_checkin banner to that page's own tablist only —
  bookings_view has a ui-tabs-nav tablist too, so it was duplicating
  alongside the existing arrival-reminder banner there
- options.js: persist whatever the default-tablet <select> ends up showing,
  not just on manual change — a <select> auto-selects its first <option>
  without firing 'change', so a single-tablet setup (or anyone happy with
  the first option) never actually got a default saved despite the dropdown
  looking selected, which sent every injected button to the manual picker

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

backend/extension-src/options/options.js

@@ -40,8 +40,19 @@ async function refreshDeviceList(preselectId) {
     defaultDeviceSelect.innerHTML = devices.map(d =>
       `<option value="${d.device_id}">${escapeHtml(d.name)}</option>`
     ).join('');
-    if (preselectId) defaultDeviceSelect.value = preselectId;
+    if (preselectId && devices.some(d => d.device_id === preselectId)) {
+      defaultDeviceSelect.value = preselectId;
+    }
     defaultSection.style.display = '';
+    // A <select> always shows *something* selected — the browser defaults to
+    // the first <option> with no explicit selection, with no 'change' event
+    // firing for that. Without this, a machine with only one tablet (or
+    // whoever's happy with the first one) never actually got a default
+    // saved, despite the dropdown visibly showing one selected — the
+    // injected buttons would then always fall back to the manual picker
+    // (confirmed live 2026-10-01). Persisting whatever ends up selected here
+    // keeps storage and the visible dropdown state from ever drifting apart.
+    chrome.storage.local.set({ defaultDeviceId: defaultDeviceSelect.value });
   } catch {
     // Not configured/reachable yet — leave the section hidden rather than
     // showing an empty/broken picker; Test Connection surfaces the real error.
@@ -83,7 +94,8 @@ async function save() {
   await chrome.storage.local.set({ baseUrl, apiKey });
   statusEl.textContent = 'Saved.';
   statusEl.style.color = '#16a34a';
-  refreshDeviceList();
+  const { defaultDeviceId } = await chrome.storage.local.get('defaultDeviceId');
+  refreshDeviceList(defaultDeviceId);
 }
 
 async function test() {
@@ -93,7 +105,8 @@ async function test() {
     const devices = await fetchDevices();
     statusEl.textContent = `Connected — found ${devices.length} tablet(s).`;
     statusEl.style.color = '#16a34a';
-    refreshDeviceList();
+    const { defaultDeviceId } = await chrome.storage.local.get('defaultDeviceId');
+    refreshDeviceList(defaultDeviceId);
   } catch (err) {
     statusEl.textContent = `Failed: ${err.message}`;
     statusEl.style.color = '#dc2626';