Sprites tab: add drag-onto-scene helper text at the top

The arrow points at the viewport to the left of the sidebar. New
.drag-hint class since .hint uses margin-top:auto and would sink to the
bottom of the tab.

Author: BinaryConstruct

src/App.css

@@ -557,6 +557,13 @@
   margin: 0;
 }
 
+/* sprites tab: the arrow points at the viewport left of the sidebar */
+.drag-hint {
+  margin: 0;
+  font-size: 12px;
+  color: #8b93a5;
+}
+
 .hint {
   margin-top: auto;
   font-size: 11px;

src/ui/SpritesTab.tsx

@@ -57,6 +57,7 @@ export function SpritesTab({ onChanged }: SpritesTabProps) {
 
   return (
     <div className="asset-tab">
+      <p className="drag-hint">← Drag onto scene</p>
       <div className="layer-panel-head">
         <h2>Your sprites</h2>
         <span className="add-buttons">