Improve: fix white scrollbar issue in Tauri dark theme

- Add color-scheme: dark meta tag and CSS for proper dark scrollbars
- Implement ultra-thin (3px) elegant scrollbars globally
- Remove overflow-hidden that was cutting scrollbar at top
- Clean up redundant scrollbar CSS and component styling
- Optimize scrollbar appearance for both web and Tauri desktop

Fixes scrollbar visibility issues in dark theme across all components.

src/components/ClaudeCodeSession.refactored.tsx

@@ -254,7 +254,7 @@ export const ClaudeCodeSession: React.FC<ClaudeCodeSessionProps> = ({
         />
 
         {/* Main content area */}
-        <div className="flex-1 flex overflow-hidden">
+        <div className="flex-1 flex">
           {showPreview ? (
             <SplitPane
               left={

src/components/ui/scroll-area.tsx

@@ -25,16 +25,7 @@ export const ScrollArea = React.forwardRef<HTMLDivElement, ScrollAreaProps>(
     return (
       <div
         ref={ref}
-        className={cn(
-          "relative overflow-auto",
-          // Custom scrollbar styling
-          "scrollbar-thin scrollbar-thumb-border scrollbar-track-transparent",
-          "[&::-webkit-scrollbar]:w-2",
-          "[&::-webkit-scrollbar-track]:bg-transparent",
-          "[&::-webkit-scrollbar-thumb]:bg-border [&::-webkit-scrollbar-thumb]:rounded-full",
-          "[&::-webkit-scrollbar-thumb:hover]:bg-border/80",
-          className
-        )}
+        className={cn("relative overflow-auto", className)}
         {...props}
       >
         {children}