fix(css): downgrade Tailwind CSS from v4 to v3.4 for better browser compatibility

Tailwind CSS v4 requires modern CSS features (@layer, @property, color-mix)
that are not supported in older WebView2 versions, causing styles to fail
on some Windows 11 systems.

Changes:
- Replace @tailwindcss/vite with postcss + autoprefixer
- Downgrade tailwindcss from 4.1.13 to 3.4.17
- Convert CSS imports from v4 syntax to v3 @tailwind directives
- Add darkMode: "selector" to tailwind.config.js
- Create postcss.config.js for PostCSS pipeline

This improves compatibility with older browsers and WebView2 runtimes
while maintaining the same visual appearance.

src/index.css

@@ -1,8 +1,7 @@
-/* 引入 Tailwind v4 内建样式与工具 */
-@import "tailwindcss";
-
-/* 覆盖 Tailwind v4 默认的 dark 变体为"类选择器"模式 */
-@custom-variant dark (&:where(.dark, .dark *));
+/* Tailwind CSS v3 指令 */
+@tailwind base;
+@tailwind components;
+@tailwind utilities;
 
 /* shadcn/ui 主题变量 - 蓝色主题 */
 @layer base {