feat(upload): support batching via header and update ShareX config

Introduce support for grouping multiple sequential file uploads into a single box using the `X-Warpbox-Batch` header. This is particularly useful for ShareX multi-file selections, which are sent as separate back-to-back requests.

Additionally, this change:
- Updates the ShareX configuration template to opt-in to batching by default.
- Switches ShareX configuration placeholders to the modern `{json:...}` format.
- Adds `thumbnailUrl` to the upload response schema and documents its usage.

examples/sharex/warpbox-anonymous.sxcu

@@ -5,10 +5,13 @@
   "RequestMethod": "POST",
   "RequestURL": "https://warpbox.dev/api/v1/upload",
   "Headers": {
-    "Accept": "application/json"
+    "Accept": "application/json",
+    "X-Warpbox-Batch": "sharex"
   },
   "Body": "MultipartFormData",
   "FileFormName": "sharex",
-  "URL": "$json:boxUrl$",
-  "DeletionURL": "$json:manageUrl$"
+  "URL": "{json:boxUrl}",
+  "ThumbnailURL": "{json:thumbnailUrl}",
+  "DeletionURL": "{json:deleteUrl}",
+  "ErrorMessage": "{json:error}"
 }