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/pages/home.html

@@ -80,6 +80,10 @@
         </div>
       </div>
       <div class="result-list" id="result-list"></div>
+      <p class="manage-link" id="manage-link" hidden>
+        <span>Keep this private:</span>
+        <a href="/" target="_blank" rel="noopener noreferrer">Manage or delete this upload</a>
+      </p>
     </div>
   </section>
 </section>