fix: 修复暗黑模式样式兼容性

- 在 tailwind.config.js 中启用 darkMode: 'class' 并扩展语义化颜色变量
- 将全项目硬编码颜色（bg-white、text-gray-*、border-gray-* 等）替换为语义化类名
- 修复 popup/index.html 硬编码浅色背景色
- 同步更新相关单元测试中的类名断言

pages/Timestamp/LiveClock.tsx

@@ -37,17 +37,17 @@ const LiveClock = React.memo(({ unit, onUseNow }: LiveClockProps) => {
   }, [now, showMessage, t]);
 
   return (
-    <div className="flex items-center gap-3 px-4 py-2 mb-4 bg-blue-50 rounded-lg border border-blue-100">
-      <span className="text-blue-700 font-bold text-[0.65rem] uppercase tracking-wider whitespace-nowrap">
+    <div className="flex items-center gap-3 px-4 py-2 mb-4 bg-primary/10 rounded-lg border border-primary/20">
+      <span className="text-primary font-bold text-[0.65rem] uppercase tracking-wider whitespace-nowrap">
         {t('timestamp:currentTs')}
       </span>
-      <span className="flex-1 font-mono font-bold text-blue-700 text-sm tracking-tight leading-tight overflow-hidden text-ellipsis">
+      <span className="flex-1 font-mono font-bold text-primary text-sm tracking-tight leading-tight overflow-hidden text-ellipsis">
         {displayVal}
       </span>
       <button
         onClick={handleUseNow}
         title={t('timestamp:useNowTooltip')}
-        className="p-1.5 rounded-md bg-white text-blue-700 shadow-sm hover:bg-blue-700 hover:text-white transition-colors"
+        className="p-1.5 rounded-md bg-background text-primary shadow-sm hover:bg-blue-700 hover:text-white transition-colors"
       >
         <Clock className="w-4 h-4" />
       </button>