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.

backend/static/css/40-docs.css

@@ -36,6 +36,22 @@
   font-size: 1rem;
 }
 
+.docs-card h3 {
+  margin: 1.35rem 0 0;
+  font-size: 0.9rem;
+  font-weight: 650;
+  color: var(--foreground);
+}
+
+/* Highlights where the API token goes in the ShareX config snippet. */
+.sxcu-highlight {
+  background: #fde047;
+  color: #1a1a1a;
+  font-weight: 700;
+  padding: 0 0.2rem;
+  border-radius: 3px;
+}
+
 .docs-card p {
   margin: 0.65rem 0 0;
   color: var(--muted-foreground);