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/manage_deleted.html

@@ -0,0 +1,16 @@
+{{define "manage_deleted.html"}}{{template "base" .}}{{end}}
+
+{{define "content"}}
+<section class="download-view" aria-labelledby="deleted-title">
+  <div class="card download-card">
+    <div class="card-content">
+      <div class="file-emblem" aria-hidden="true">
+        <svg viewBox="0 0 24 24" role="img" focusable="false"><path d="M20 6 9 17l-5-5" /></svg>
+      </div>
+      <h1 id="deleted-title">Upload deleted</h1>
+      <p class="download-subtitle">Box <code>{{.Data.ID}}</code> and all of its files have been removed.</p>
+      <a class="button button-primary button-wide" href="/">Upload another file</a>
+    </div>
+  </div>
+</section>
+{{end}}