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/Base64Converter/TextMode.tsx

@@ -120,12 +120,12 @@ export default function TextMode({ onSwitchToImageMode }: TextModeProps = {}) {
       />
 
       {showImageHint && (
-        <div className="flex items-center justify-between p-3 rounded-lg bg-blue-50 border border-blue-200">
-          <span className="text-sm text-blue-700">{t('imageDataUriHint')}</span>
+        <div className="flex items-center justify-between p-3 rounded-lg bg-primary/10 border border-primary/30">
+          <span className="text-sm text-primary">{t('imageDataUriHint')}</span>
           <button
             type="button"
             onClick={onSwitchToImageMode}
-            className="px-3 py-1 text-sm font-medium text-blue-600 hover:bg-blue-100 rounded-md transition-colors"
+            className="px-3 py-1 text-sm font-medium text-primary hover:bg-blue-100 rounded-md transition-colors"
           >
             {t('switchToImageMode')}
           </button>
@@ -133,9 +133,9 @@ export default function TextMode({ onSwitchToImageMode }: TextModeProps = {}) {
       )}
 
       {output && (
-        <div className="p-4 rounded-xl bg-blue-50 border border-blue-200">
+        <div className="p-4 rounded-xl bg-primary/10 border border-primary/30">
           <div className="flex justify-between items-center mb-2">
-            <span className="text-xs font-bold text-gray-500">{outputLabel}</span>
+            <span className="text-xs font-bold text-muted-foreground">{outputLabel}</span>
             <CopyButton text={output} />
           </div>
           <TextInputArea