feat(web): MobileAppDownload component on login + settings (#397 follow-up)

Closes the discoverability gap on the in-app update flow — the
/api/client/apk endpoint exists but had no web UI surface to find
it. User asked to be able to "visit the site on my phone and
download the apk from there."

- web/src/lib/components/MobileAppDownload.svelte — fetches
  /api/client/version once on mount; renders a download link with
  version + size when 200; renders nothing on 404 (no APK bundled,
  graceful degradation in dev environments and pre-CI-wiring images).
- Mounted on the login page (below the Register link) so the link
  is discoverable without authentication. The /api/client/* endpoints
  are themselves unauthed, so the flow works end-to-end for any
  visitor on a phone.
- Also mounted in Settings → Mobile app section for logged-in users
  who want to grab the matching APK for sideloading on a different
  device.

Browser handles the download via the server's existing
Content-Disposition: attachment; filename="minstrel.apk" header.

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

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

@@ -4,6 +4,7 @@
   import { goto } from '$app/navigation';
   import { login } from '$lib/auth/store.svelte';
   import type { ApiError } from '$lib/api/client';
+  import MobileAppDownload from '$lib/components/MobileAppDownload.svelte';
 
   let username = $state('');
   let password = $state('');
@@ -88,5 +89,9 @@
       Don't have an account?
       <a href="/register" class="text-accent hover:underline">Register</a>
     </p>
+
+    <div class="mt-6">
+      <MobileAppDownload />
+    </div>
   </div>
 </main>

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

@@ -17,6 +17,7 @@
   } from '$lib/api/me';
   import { errCode } from '$lib/api/errors';
   import { pushToast } from '$lib/stores/toast.svelte';
+  import MobileAppDownload from '$lib/components/MobileAppDownload.svelte';
 
   const queryClient = useQueryClient();
 
@@ -323,4 +324,12 @@
       </li>
     </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>
 </div>