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
       });
     }
   }, []);

components/editor/tool-options.tsx

@@ -12,9 +12,12 @@ export function ToolOptions() {
   // Drawing tools: brush, pencil, eraser
   const isDrawingTool = ['brush', 'eraser', 'pencil'].includes(activeTool);
   const showHardness = ['brush'].includes(activeTool);
-  const showColor = ['brush', 'pencil', 'fill'].includes(activeTool);
+  const showColor = ['brush', 'pencil'].includes(activeTool);
   const showFlow = ['brush'].includes(activeTool);
 
+  // Fill tool
+  const isFillTool = activeTool === 'fill';
+
   // Shape tool
   const isShapeTool = activeTool === 'shape';
 
@@ -22,7 +25,7 @@ export function ToolOptions() {
   const isSelectionTool = activeTool === 'select';
 
   // Don't show options bar if no options available
-  if (!isDrawingTool && !isShapeTool && !isSelectionTool) {
+  if (!isDrawingTool && !isFillTool && !isShapeTool && !isSelectionTool) {
     return null;
   }
 
@@ -125,6 +128,46 @@ export function ToolOptions() {
         </>
       )}
 
+      {/* Fill Tool Options */}
+      {isFillTool && (
+        <>
+          <div className="flex items-center gap-2">
+            <label className="text-sm font-medium text-card-foreground whitespace-nowrap">
+              Color:
+            </label>
+            <input
+              type="color"
+              value={settings.color}
+              onChange={(e) => setColor(e.target.value)}
+              className="h-8 w-16 rounded border border-border cursor-pointer"
+            />
+            <input
+              type="text"
+              value={settings.color}
+              onChange={(e) => setColor(e.target.value)}
+              className="w-24 px-2 py-1 text-xs rounded border border-border bg-background text-foreground"
+            />
+          </div>
+
+          <div className="flex items-center gap-2">
+            <label className="text-sm font-medium text-card-foreground whitespace-nowrap">
+              Opacity:
+            </label>
+            <input
+              type="range"
+              min="0"
+              max="100"
+              value={settings.opacity * 100}
+              onChange={(e) => setOpacity(Number(e.target.value) / 100)}
+              className="w-32"
+            />
+            <span className="text-sm text-muted-foreground w-10">
+              {Math.round(settings.opacity * 100)}%
+            </span>
+          </div>
+        </>
+      )}
+
       {/* Shape Tool Options */}
       {isShapeTool && (
         <div className="flex items-center gap-2">