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>

index.html

@@ -605,6 +605,18 @@
           <div id="s-cover-color-bg" class="toggle" onclick="this.classList.toggle('on')"></div>
         </div>
       </div>
+      <div class="settings-section" id="s-apk-section" style="display:none">
+        <h3 style="display:flex;align-items:center;gap:8px;margin-bottom:12px">
+          <span style="width:24px;height:24px;border-radius:6px;background:var(--surface2);display:inline-flex;align-items:center;justify-content:center;font-size:12px">🤖</span>
+          Android App
+        </h3>
+        <div style="font-size:12px;color:var(--text2);margin-bottom:10px;line-height:1.6">Installiere die NovaSonic Android-App direkt aufs Handy. <span id="s-apk-meta" style="color:var(--text3)"></span></div>
+        <a id="s-apk-btn" href="#" download style="display:inline-flex;align-items:center;gap:8px;padding:10px 16px;border-radius:10px;background:linear-gradient(135deg,var(--accent),var(--accent-dark));color:#fff;font-weight:700;font-size:13px;text-decoration:none;box-shadow:0 4px 12px var(--accent-glow)">
+          <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><path d="M21 15v4a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2v-4"/><polyline points="7 10 12 15 17 10"/><line x1="12" y1="15" x2="12" y2="3"/></svg>
+          APK herunterladen
+        </a>
+        <div style="font-size:11px;color:var(--text3);margin-top:8px">Danach im Handy „Installation aus unbekannten Quellen" für den Browser erlauben.</div>
+      </div>
       <div class="settings-section">
         <h3 style="display:flex;align-items:center;gap:8px;margin-bottom:12px">
           <span style="width:24px;height:24px;border-radius:6px;background:var(--surface2);display:inline-flex;align-items:center;justify-content:center;font-size:12px">📊</span>