build: 前端改用 Bun + Vite，格式化改用 bun fmt

- 包管理和运行时换成 Bun：bun.lock 取代 package-lock.json，脚本用 bun --bun 运行，
  CI 改用 oven-sh/setup-bun（1.4.4，与后端镜像一致）
- 构建工具从 Rsbuild 换成 Vite 8，沿用 PUBLIC_ 环境变量前缀和 3000 端口；
  浏览器目标保持 Chrome 87 起（机房是 Chrome 105），用 plugin-legacy 按需注入
  core-js（toSorted 等），不生成旧浏览器的兼容包
- fmt 改用 Bun 自带的格式化，配置从 .prettierrc.toml 移到 bunfig.toml；
  prettier 包仍保留，编辑器里的「整理」按钮在浏览器里用它

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

client/package.json

@@ -4,10 +4,10 @@
   "version": "0.3.0",
   "type": "module",
   "scripts": {
-    "start": "rsbuild dev",
-    "build": "rsbuild build",
-    "build:staging": "rsbuild build --env-mode staging",
-    "fmt": "prettier --write src/"
+    "start": "bun --bun vite",
+    "build": "bun --bun vite build",
+    "build:staging": "bun --bun vite build --mode staging",
+    "fmt": "bun fmt src/"
   },
   "dependencies": {
     "@codemirror/lang-css": "^6.3.1",
@@ -31,12 +31,12 @@
   },
   "devDependencies": {
     "@iconify/vue": "^5.0.1",
-    "@rsbuild/core": "^2.1.13",
-    "@rsbuild/plugin-vue": "^2.0.1",
+    "@vitejs/plugin-legacy": "^8.2.3",
+    "@vitejs/plugin-vue": "^6.0.10",
     "@vue/tsconfig": "^0.9.1",
-    "core-js": "^3.50.0",
     "prettier": "^3.9.6",
     "typescript": "^7.0.2",
-    "unplugin-vue-components": "^32.1.0"
+    "unplugin-vue-components": "^32.1.0",
+    "vite": "^8.3.4"
   }
 }