Show update-available link in the sidebar, not only in Settings

The update card lived inside Settings, easy to miss on an app people
mostly leave on the Dashboard or Timing view. A small link now sits
next to the version number in the sidebar footer, on every view, and
appears automatically from the same startup update check Settings
already used - no new endpoint. Clicking it jumps straight into
Settings with the update ready to run.

Verified with a real headless-browser run against a throwaway git
clone: the link appears on its own without ever opening Settings,
names the real new version, survives navigating to another view, and
clicking it lands on Settings with the update already available there.

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

index.html

@@ -32,6 +32,7 @@
           <p id="connectionBadge" class="badge badge-offline">Decoder Offline</p>
           <small id="clock"></small>
           <small id="appVersionLabel" class="app-version" hidden></small>
+          <button id="appUpdateAvailable" class="app-update-link" type="button" hidden></button>
         </div>
       </aside>