fix(ThemeModeProvider): 修复主题快照同步与首屏闪烁问题

抽取 themeSnapshot 工具模块，异步恢复与跨窗同步时写回 snapshot；HTML 内联脚本与 useLayoutEffect 消除 dark 类首帧闪烁；避免 storage 恢复覆盖用户操作及 setState 嵌套更新。

Co-authored-by: Cursor <cursoragent@cursor.com>

src/entrypoints/popup/index.html

@@ -5,6 +5,19 @@
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>Testing Tools - 标签页</title>
     <meta name="manifest.type" content="browser_action" />
+    <script>
+      (function () {
+        try {
+          var raw = localStorage.getItem('snapshot/app/themeMode');
+          var mode = raw ? JSON.parse(raw) : 'system';
+          if (mode !== 'light' && mode !== 'dark' && mode !== 'system') mode = 'system';
+          var isDark =
+            mode === 'dark' ||
+            (mode === 'system' && window.matchMedia('(prefers-color-scheme: dark)').matches);
+          if (isDark) document.documentElement.classList.add('dark');
+        } catch (_error) {}
+      })();
+    </script>
     <style>
       /* Force initial popup size before React hydration */
       html, body {