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>

app/globals.css

@@ -156,9 +156,10 @@
     font-feature-settings: "rlig" 1, "calt" 1;
   }
 
-  /* Apply custom scrollbar globally */
+  /* Apply custom scrollbar globally with primary color accent */
   * {
     scrollbar-width: thin;
+    scrollbar-color: color-mix(in oklch, var(--primary) 40%, transparent) var(--muted);
   }
 
   *::-webkit-scrollbar {
@@ -172,18 +173,18 @@
   }
 
   *::-webkit-scrollbar-thumb {
-    background: color-mix(in oklch, var(--muted-foreground) 30%, transparent);
+    background: color-mix(in oklch, var(--primary) 40%, transparent);
     border-radius: 5px;
     border: 2px solid var(--muted);
     transition: background 0.2s ease;
   }
 
   *::-webkit-scrollbar-thumb:hover {
-    background: color-mix(in oklch, var(--muted-foreground) 50%, transparent);
+    background: color-mix(in oklch, var(--primary) 60%, transparent);
   }
 
   *::-webkit-scrollbar-thumb:active {
-    background: color-mix(in oklch, var(--muted-foreground) 70%, transparent);
+    background: color-mix(in oklch, var(--primary) 80%, transparent);
   }
 
   /* Scrollbar corners */