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/lib/components/MobileAppDownload.svelte

@@ -0,0 +1,54 @@
+<script lang="ts">
+  import { onMount } from 'svelte';
+  import { Smartphone } from 'lucide-svelte';
+
+  // Polls /api/client/version once on mount. Renders nothing if the
+  // server has no APK bundled (404) — graceful degradation in dev
+  // environments and pre-CI-wiring images.
+  //
+  // Mounted on the login page so the download is discoverable without
+  // authentication; the /api/client/* endpoints are themselves unauthed.
+  // Also reusable in Settings for logged-in users who want to grab the
+  // matching APK for sideloading on a different device.
+
+  type Info = { version: string; apkUrl: string; sizeBytes: number };
+
+  let info = $state<Info | null>(null);
+
+  function formatSize(bytes: number): string {
+    if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(0)} KB`;
+    return `${(bytes / 1024 / 1024).toFixed(1)} MB`;
+  }
+
+  onMount(async () => {
+    try {
+      const res = await fetch('/api/client/version');
+      if (!res.ok) return; // 404 = no bundled APK; stay hidden
+      const body = (await res.json()) as Partial<Info>;
+      if (!body.version || !body.apkUrl) return;
+      info = {
+        version: body.version,
+        apkUrl: body.apkUrl,
+        sizeBytes: body.sizeBytes ?? 0
+      };
+    } catch {
+      // Network error / non-JSON response — silent fail, no banner.
+    }
+  });
+</script>
+
+{#if info}
+  <a
+    href={info.apkUrl}
+    download="minstrel.apk"
+    class="mobile-app-download flex items-center gap-3 rounded-md border border-border bg-surface px-3 py-2 text-sm text-text-primary no-underline transition-colors hover:bg-surface-hover hover:text-accent focus-visible:ring-2 focus-visible:ring-accent"
+  >
+    <Smartphone size={18} strokeWidth={1.5} class="shrink-0 text-accent" />
+    <span class="flex flex-col leading-tight">
+      <span class="font-medium">Get the Android app</span>
+      <span class="text-xs text-text-secondary">
+        {info.version}{info.sizeBytes > 0 ? ` · ${formatSize(info.sizeBytes)}` : ''}
+      </span>
+    </span>
+  </a>
+{/if}