seo(site): add /download/ page (transactional intent) + internal links + sitemap

Targets 'download opencode android', 'opencode apk', 'opencode f-droid'.
Serves the live install channels (self-hosted F-Droid repo + signed APK)
with SoftwareApplication + BreadcrumbList structured data. Linked from
landing hero, landing footer, and guide footer.

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

docs-site/index.html

@@ -250,8 +250,8 @@ footer a{margin:0 8px}
     <h1>OpenCode Mobile</h1>
     <p class="tagline">The free, open-source Android client for the OpenCode AI coding agent. Connect to your own self-hosted OpenCode server and write, review, and ship code from your phone.</p>
     <div class="cta">
-      <a class="btn btn-primary" href="https://dzianisv.github.io/opencode-mobile/fdroid/repo">Get it on F-Droid</a>
-      <a class="btn" href="https://github.com/dzianisv/opencode-mobile/releases/latest">Download APK</a>
+      <a class="btn btn-primary" href="download/">Download for Android</a>
+      <a class="btn" href="https://dzianisv.github.io/opencode-mobile/fdroid/repo">Get it on F-Droid</a>
       <a class="btn" href="guide/">Setup guide</a>
       <a class="btn" href="https://github.com/dzianisv/opencode-mobile">View on GitHub</a>
     </div>
@@ -359,6 +359,7 @@ OPENCODE_SERVER_PASSWORD=yourpassword opencode serve --hostname 0.0.0.0 --port 4
 <footer>
   <div class="wrap">
     <p>
+      <a href="download/">Download</a>
       <a href="guide/">Setup guide</a>
       <a href="https://github.com/dzianisv/opencode-mobile">GitHub</a>
       <a href="https://dzianisv.github.io/opencode-mobile/fdroid/repo">F-Droid repo</a>