fix(web): collapse the entire Mobile app section when no APK bundled

When /api/client/version returns 404 (no APK in /app/client/, e.g.
v2026.05.10.0 which had a failed CI APK-attach step), the previous
shape rendered the section heading + intro paragraph with an empty
hole where the button should be. Confusing — looks like a broken
button.

Moved the section wrapper, heading, and intro paragraph INSIDE
MobileAppDownload so the whole block collapses together when info
is null. Settings page just mounts <MobileAppDownload />; nothing
visible if no APK is available.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/routes/settings/+page.svelte

@@ -325,11 +325,5 @@
     </ul>
   </section>
 
-  <section class="space-y-3 rounded border border-border bg-surface p-4">
-    <h2 class="text-lg font-semibold">Mobile app</h2>
-    <p class="text-sm text-text-secondary">
-      Install the Android app paired to this server. Sign in with the same account.
-    </p>
-    <MobileAppDownload />
-  </section>
+  <MobileAppDownload />
 </div>