Extension: show installed/latest version for drift tracking

Popup footer shows its own installed version (from manifest.json, via
chrome.runtime.getManifest()); Terminals page shows the latest bundled
version next to the download link (new GET /api/extension/version, reads
the same manifest.json). Bumped manifest.json to 0.2.0 for the Today's
Arrivals change, and documented in extension-src/README.md that every
future change to this folder must bump the version too — nothing else
keeps it in sync automatically.

backend/extension-src/popup/popup.css

@@ -97,3 +97,7 @@ a.settings-link { display: block; margin-top: 12px; font-size: 12px; color: #7b4
   padding: 4px 10px; border-radius: 6px; cursor: pointer;
 }
 .send-small:disabled { opacity: 0.5; cursor: default; }
+
+.version {
+  margin-top: 10px; text-align: center; font-size: 10px; color: #94a3b8;
+}

backend/extension-src/popup/popup.html

@@ -15,6 +15,8 @@
     <p class="muted">Loading…</p>
   </div>
 
+  <div id="version" class="version"></div>
+
   <script type="module" src="popup.js"></script>
 </body>
 </html>

backend/extension-src/popup/popup.js

@@ -9,6 +9,15 @@ const BOOKING_URL_RE = /\/bookings_(?:view|edit|checkin)\/(\d+)/i;
 
 const contentEl = document.getElementById('content');
 const nbStatusEl = document.getElementById('nbStatus');
+const versionEl = document.getElementById('version');
+
+// Shown so staff/admin can tell at a glance whether a given reception PC is
+// running an old install — compare against the "Latest version" shown next
+// to the download link on tablet-checkin's own Terminals page (see
+// routes/extension.js's /api/extension/version). Pulled from the manifest
+// rather than hardcoded here, so it can never itself drift out of sync with
+// manifest.json's own version field.
+versionEl.textContent = `v${chrome.runtime.getManifest().version}`;
 
 async function main() {
   const [tab] = await chrome.tabs.query({ active: true, currentWindow: true });