chore: Tailwind CSS 3.4 迁移到 4.x（CSS-first 配置 + @tailwindcss/vite）

- 升级 tailwindcss@4.3 + @tailwindcss/vite，移除 postcss/autoprefixer/tailwindcss-animate
- tailwindcss-animate 替换为 tw-animate-css（v4 兼容的 animate-in/out 工具类）
- global.css 改用 @import "tailwindcss" + @theme inline 映射 shadcn 设计令牌
- @custom-variant dark 替代 darkMode:'class'；@source 显式纳入 pages/ 目录
- 删除 tailwind.config.ts / postcss.config.js，components.json 同步 v4 约定
- vite.config.ts 接入 tailwindcss 插件，__dirname 改 import.meta.dirname

frontend/package.json

@@ -22,6 +22,7 @@
     "@radix-ui/react-switch": "^1.3.0",
     "@svg-maps/china": "^2.0.0",
     "@svg-maps/world": "^2.0.0",
+    "@tailwindcss/vite": "^4.3.3",
     "@tanstack/react-virtual": "^3.11.2",
     "@tiptap/core": "^3.26.1",
     "@tiptap/extension-code-block": "^3.26.1",
@@ -33,7 +34,6 @@
     "@tiptap/pm": "^3.26.1",
     "@tiptap/react": "^3.26.1",
     "@tiptap/starter-kit": "^3.26.1",
-    "autoprefixer": "^10.5.0",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "compression": "^1.8.1",
@@ -46,7 +46,6 @@
     "linkedom": "^0.18.13",
     "lucide-react": "^1.18.0",
     "marked": "^18.0.5",
-    "postcss": "^8.5.15",
     "react": "^19.2.8",
     "react-dom": "^19.2.8",
     "react-easy-crop": "^6.0.2",
@@ -55,9 +54,9 @@
     "react-streaming": "^0.4.20",
     "sonner": "^2.0.7",
     "tailwind-merge": "^3.6.0",
-    "tailwindcss": "^3.4.19",
-    "tailwindcss-animate": "^1.0.7",
+    "tailwindcss": "^4.3.3",
     "turndown": "^7.2.4",
+    "tw-animate-css": "^1.4.0",
     "vike": "^0.4.266",
     "zod": "^4.4.3",
     "zustand": "^5.0.15"