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>

store/canvas-store.ts

@@ -58,7 +58,7 @@ export const useCanvasStore = create<CanvasStore>()(
       zoom: 1,
       offsetX: 0,
       offsetY: 0,
-      backgroundColor: '#ffffff',
+      backgroundColor: 'transparent',
       showGrid: false,
       gridSize: 20,
       showRulers: true,