refactor: 全面重构前端样式，消除内联 style 和硬编码色值，统一 TDesign 规范

- 重写 styles.css：CSS 变量化可用率色阶、状态色类、工具类、安全选择器
- 组件改造：StatusDot/StatusBar/TargetDetailDrawer/GroupHeader 等改用 CSS 类和 Typography
- color-threshold 移除 getLatencyColor 死代码，保留 getAvailabilityProgressColor 返回 CSS 变量
- target-table-columns 状态列和延迟列切换为 CSS 类
- 新增 css-utility-classes spec，更新 4 个 main specs（probe/card/table/drawer）
- README 和 config.yaml 写入前端样式开发规范

src/web/app.tsx

@@ -1,4 +1,4 @@
-import { Alert, Loading } from "tdesign-react";
+import { Alert, Loading, Typography } from "tdesign-react";
 import { useSummary, useTargets, useTargetDetail } from "./hooks/useTargetDetail";
 import { SummaryCards } from "./components/SummaryCards";
 import { TargetBoard } from "./components/TargetBoard";
@@ -26,8 +26,8 @@ export function App() {
   return (
     <main className="dashboard">
       <header className="dashboard-header">
-        <h1>DiAL</h1>
-        <p className="dashboard-subtitle">统一拨测平台</p>
+        <Typography.Title level="h1">DiAL</Typography.Title>
+        <Typography.Text theme="secondary">统一拨测平台</Typography.Text>
       </header>
 
       {error && <Alert theme="error" message={`请求失败: ${error.message}`} closeBtn />}