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.html

@@ -0,0 +1,32 @@
+{{define "manage.html"}}{{template "base" .}}{{end}}
+
+{{define "content"}}
+<section class="download-view" aria-labelledby="manage-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="M3 6h18" /><path d="M8 6V4h8v2" /><path d="M19 6l-1 14H6L5 6" /><path d="M10 11v5" /><path d="M14 11v5" /></svg>
+      </div>
+      <h1 id="manage-title">Manage upload</h1>
+      <p class="download-subtitle">This private link can delete the entire box.</p>
+
+      <dl class="manage-details">
+        <div><dt>Box</dt><dd><code>{{.Data.Box.ID}}</code></dd></div>
+        <div><dt>Files</dt><dd>{{.Data.FileCount}}</dd></div>
+        <div><dt>Total size</dt><dd>{{.Data.TotalSize}}</dd></div>
+        <div><dt>Expires</dt><dd>{{.Data.ExpiresLabel}}</dd></div>
+        <div><dt>Downloads</dt><dd>{{.Data.DownloadCount}}{{if .Data.MaxDownloads}} / {{.Data.MaxDownloads}}{{end}}</dd></div>
+        <div><dt>Protected</dt><dd>{{if .Data.Protected}}Yes{{else}}No{{end}}</dd></div>
+      </dl>
+
+      <form action="{{.Data.DeleteActionURL}}" method="post">
+        <button class="button button-danger button-wide" type="submit">
+          <svg viewBox="0 0 24 24" role="img" focusable="false" aria-hidden="true"><path d="M3 6h18" /><path d="M8 6V4h8v2" /><path d="M19 6l-1 14H6L5 6" /></svg>
+          Delete upload
+        </button>
+      </form>
+      <a class="button button-outline button-wide" href="/d/{{.Data.Box.ID}}">Back to box</a>
+    </div>
+  </div>
+</section>
+{{end}}