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.js

@@ -1,12 +1,62 @@
 const baseUrlInput = document.getElementById('baseUrl');
 const apiKeyInput = document.getElementById('apiKey');
 const statusEl = document.getElementById('status');
+const defaultSection = document.getElementById('defaultTabletSection');
+const defaultDeviceSelect = document.getElementById('defaultDevice');
 
 async function load() {
-  const { baseUrl, apiKey } = await chrome.storage.local.get(['baseUrl', 'apiKey']);
+  const { baseUrl, apiKey, defaultDeviceId } = await chrome.storage.local.get(['baseUrl', 'apiKey', 'defaultDeviceId']);
   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.
+  if (baseUrl && apiKey) refreshDeviceList(defaultDeviceId);
+}
+
+// Shared by Test Connection and the default-tablet picker — one fetch, two
+// uses, rather than hitting the API twice for what's the same data.
+async function fetchDevices() {
+  const { baseUrl, apiKey } = await chrome.storage.local.get(['baseUrl', 'apiKey']);
+  if (!baseUrl || !apiKey) throw new Error('Set both fields and save first.');
+  const res = await fetch(`${baseUrl}/api/staff/devices`, {
+    headers: { Authorization: `Bearer ${apiKey}` },
+  });
+  const body = await res.json().catch(() => ([]));
+  if (!res.ok) throw new Error(body.error || `HTTP ${res.status}`);
+  return body;
+}
+
+// Populates the "default tablet for this machine" picker — see project_checkin
+// memory's extension notes: the popup pre-selects this but still lets staff
+// override it; the injected buttons skip asking entirely and push straight
+// to it unless it's busy. Selection auto-saves on change, no separate button
+// — it's a small enough setting not to warrant one.
+async function refreshDeviceList(preselectId) {
+  try {
+    const devices = await fetchDevices();
+    if (!devices.length) {
+      defaultSection.style.display = 'none';
+      return;
+    }
+    defaultDeviceSelect.innerHTML = devices.map(d =>
+      `<option value="${d.device_id}">${escapeHtml(d.name)}</option>`
+    ).join('');
+    if (preselectId) defaultDeviceSelect.value = preselectId;
+    defaultSection.style.display = '';
+  } catch {
+    // Not configured/reachable yet — leave the section hidden rather than
+    // showing an empty/broken picker; Test Connection surfaces the real error.
+    defaultSection.style.display = 'none';
+  }
+}
+
+defaultDeviceSelect.addEventListener('change', () => {
+  chrome.storage.local.set({ defaultDeviceId: defaultDeviceSelect.value });
+});
+
+function escapeHtml(s) {
+  const div = document.createElement('div');
+  div.textContent = s;
+  return div.innerHTML;
 }
 
 // tablet-checkin's own base URL isn't known in advance (dev today, a
@@ -33,28 +83,17 @@ async function save() {
   await chrome.storage.local.set({ baseUrl, apiKey });
   statusEl.textContent = 'Saved.';
   statusEl.style.color = '#16a34a';
+  refreshDeviceList();
 }
 
 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();
+    const devices = await fetchDevices();
     statusEl.textContent = `Connected — found ${devices.length} tablet(s).`;
     statusEl.style.color = '#16a34a';
+    refreshDeviceList();
   } catch (err) {
     statusEl.textContent = `Failed: ${err.message}`;
     statusEl.style.color = '#dc2626';