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/JsonTools/DiffNavigator.tsx

@@ -14,14 +14,14 @@ export default function DiffNavigator({ total, currentIndex, onPrev, onNext }: D
 
   if (total === 0) {
     return (
-      <div className="flex items-center justify-center gap-3 p-2.5 rounded-lg bg-blue-50 border border-blue-200">
-        <span className="text-sm font-bold text-gray-500">{t('jsonDiff:noDiffs')}</span>
+      <div className="flex items-center justify-center gap-3 p-2.5 rounded-lg bg-primary/10 border border-primary/30">
+        <span className="text-sm font-bold text-muted-foreground">{t('jsonDiff:noDiffs')}</span>
       </div>
     );
   }
 
   return (
-    <div className="flex items-center justify-center gap-3 p-2.5 rounded-lg bg-blue-50 border border-blue-200">
+    <div className="flex items-center justify-center gap-3 p-2.5 rounded-lg bg-primary/10 border border-primary/30">
       <button
         type="button"
         aria-label={t('jsonDiff:previousDiff')}