Show channel count for "Без категории" on the categories management page

Reuses the existing GET /api/channels?uncategorized=true endpoint client-side
rather than changing the /api/categories response shape, which stays a
plain array (kept several existing call sites simple).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/App.css

@@ -239,6 +239,16 @@
   padding: 8px 12px;
 }
 
+.category-row-virtual {
+  background: #f7f7f7;
+  border-style: dashed;
+  color: #888;
+}
+
+.category-row-virtual .reorder-buttons {
+  width: 14px;
+}
+
 .reorder-buttons {
   display: flex;
   flex-direction: column;