feat: improve UI and transparency support

UI Improvements:
- Style scrollbars with primary color accent
- Scrollbar thumb transitions on hover (40% → 60% → 80% opacity)
- Add fill tool options to toolbar (color picker + opacity)
- Support for Firefox with scrollbar-color property

Transparency Support:
- Set default canvas background to transparent
- First layer now transparent instead of white fill
- Enables creating images with transparency
- Checkerboard pattern shows through transparent areas
- Proper PNG export support with alpha channel

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

components/editor/editor-layout.tsx

@@ -37,10 +37,10 @@ export function EditorLayout() {
     if (layers.length === 0) {
       const { createLayer } = useLayerStore.getState();
       createLayer({
-        name: 'Background',
+        name: 'Layer 1',
         width: 800,
         height: 600,
-        fillColor: '#ffffff',
+        // No fillColor - layer is transparent by default
       });
     }
   }, []);