seo(site): add /features/ page (capabilities, use cases, comparison)

Targets "opencode mobile features", "AI coding android", and
"opencode vs ChatGPT for coding" queries not covered by existing pages.
Includes SoftwareApplication + BreadcrumbList structured data.
Adds /features/ to sitemap and internal footer nav on all 5 pages.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs-site/index.html

@@ -279,6 +279,7 @@ footer a{margin:0 8px}
       <li><b>Secure credential storage</b><span>Server credentials stored in the Android Keystore via expo-secure-store.</span></li>
       <li><b>Session management</b><span>Browse, create, and resume coding sessions on your server.</span></li>
     </ul>
+    <p style="margin-top:16px"><a href="features/">Full feature list and use cases →</a></p>
   </div>
 </section>
 
@@ -361,6 +362,7 @@ OPENCODE_SERVER_PASSWORD=yourpassword opencode serve --hostname 0.0.0.0 --port 4
     <p>
       <a href="download/">Download</a>
       <a href="guide/">Setup guide</a>
+      <a href="features/">Features</a>
       <a href="troubleshooting/">Troubleshooting</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>