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.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>