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.

README.md

@@ -54,6 +54,30 @@ go run ./cmd/warpbox
 - Expired boxes and boxes that have reached their download limit are cleaned on startup and then every `WARPBOX_CLEANUP_EVERY` when `WARPBOX_CLEANUP_ENABLED=true`.
 - Missing image/video thumbnails are generated in a background worker every `WARPBOX_THUMBNAIL_EVERY` when `WARPBOX_THUMBNAIL_ENABLED=true`.
 
+## Stage 3 Anonymous Integrations
+
+Anonymous uploads now return a private management link at creation time. Keep that link secret:
+anyone with it can delete the entire upload box. The raw delete token is not stored and cannot be
+recovered later.
+
+Browser uploads still show `Open box` and `Copy URL` as the primary actions, with a smaller
+`Manage or delete this upload` link in the completion panel.
+
+Curl and custom uploaders can use the same endpoint:
+
+```bash
+# Terminal-friendly output: one plain box URL.
+curl -F file=@./report.pdf http://localhost:8080/api/v1/upload
+
+# JSON output with boxUrl, manageUrl, deleteUrl, zipUrl, and file entries.
+curl -F sharex=@./screenshot.png \
+  -H 'Accept: application/json' \
+  http://localhost:8080/api/v1/upload
+```
+
+The upload endpoint accepts multipart fields named `file` and `sharex`. ShareX users can start
+from `examples/sharex/warpbox-anonymous.sxcu`; update `RequestURL` to match your instance URL.
+
 ## Runtime Data
 
 Warpbox keeps local runtime data under the configured data directory: