feat(web): download the Android APK from Settings

Backend serves a staged APK so co-users self-serve the latest build:
- GET /api/apk streams APK_PATH (default /data/novasonic.apk) with the
  android package content-type + attachment disposition; 404 if absent.
- GET /api/apk/info reports availability, size, mtime, and an optional
  version from /data/novasonic.version.txt.
- Settings gets a "🤖 Android App" section that appears only when an APK is
  present and shows version/size/date.

Maintainer updates the APK with:
  docker cp app-release.apk novasonic-backend:/data/novasonic.apk

Android source is already tracked; only build artifacts stay out of git.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/settings.js

@@ -54,6 +54,27 @@ export function openSettings() {
   _renderThemePicker();
   document.getElementById('settings-modal').classList.add('open');
   checkBackend();
+  _loadApkInfo();
+}
+
+// Show the "download the Android APK" button only if the backend actually has an APK staged.
+async function _loadApkInfo() {
+  const section = document.getElementById('s-apk-section');
+  const btn  = document.getElementById('s-apk-btn');
+  const meta = document.getElementById('s-apk-meta');
+  if (!section || !btn) return;
+  section.style.display = 'none';
+  const base = S.cfg.backendUrl;
+  if (!base) return;
+  try {
+    const info = await fetch(`${base}/api/apk/info`).then(r => r.json());
+    if (!info?.available) return;
+    btn.href = `${base}/api/apk`;
+    const mb   = info.size ? (info.size / 1048576).toFixed(1) + ' MB' : '';
+    const date = info.updatedAt ? new Date(info.updatedAt).toLocaleDateString('de-DE') : '';
+    if (meta) meta.textContent = [info.version && `v${info.version}`, mb, date && `Stand ${date}`].filter(Boolean).join(' · ');
+    section.style.display = '';
+  } catch {}
 }
 
 function _renderThemePicker() {