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.

index.html

@@ -4,6 +4,7 @@
     <meta charset="UTF-8" />
     <link rel="icon" type="image/svg+xml" href="/vite.svg" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <meta name="color-scheme" content="dark" />
     <title>Claudia - Claude Code Session Browser</title>
   </head>