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/static/js/app.js

@@ -12,6 +12,7 @@
   const totalProgressBar = document.querySelector("#total-progress-bar");
   const copyURL = document.querySelector("#copy-url");
   const openBox = document.querySelector("#open-box");
+  const manageLink = document.querySelector("#manage-link");
   const fileBrowser = document.querySelector("[data-file-browser]");
   const viewButtons = document.querySelectorAll("[data-view-button]");
   const previewImages = document.querySelector("[data-preview-images]");
@@ -228,6 +229,13 @@
     result.hidden = false;
     openBox.href = payload.boxUrl;
     resultMeta.textContent = `${payload.files.length} file${payload.files.length === 1 ? "" : "s"} · expires ${formatDate(payload.expiresAt)}`;
+    if (manageLink) {
+      const anchor = manageLink.querySelector("a");
+      manageLink.hidden = !payload.manageUrl;
+      if (anchor && payload.manageUrl) {
+        anchor.href = payload.manageUrl;
+      }
+    }
 
     resultList.replaceChildren();
     payload.files.forEach((file) => {