feat(api): add API documentation and ShareX integration

- Add an API documentation page with curl and ShareX examples.
- Implement a dynamic ShareX configuration endpoint (`/api/v1/sharex/warpbox-anonymous.sxcu`) that generates a `.sxcu` file pre-configured with the instance's base URL.
- Update anonymous uploads to return a private management link (`manageUrl`) and a deletion link (`deleteUrl`) in JSON responses.
- Update README with details on Stage 3 Anonymous Integrations.
- Add styling for the new API documentation view and management details.

backend/templates/layouts/base.html

@@ -27,7 +27,7 @@
         <span>{{.AppName}}</span>
       </a>
       <div class="nav-links">
-        <a class="button button-ghost" href="/api/v1/health">API</a>
+        <a class="button button-ghost" href="/api">API</a>
         <a class="button button-outline" href="/healthz">Health</a>
       </div>
     </nav>