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/utils/__tests__/themeSnapshot.test.ts

@@ -0,0 +1,41 @@
+import {
+  applyResolvedThemeClass,
+  applyThemeFromSnapshot,
+  getThemeSyncSnapshot,
+  persistThemeModeSnapshot,
+  resolveThemeMode,
+  THEME_MODE_SNAPSHOT_KEY,
+} from '@/utils/themeSnapshot';
+
+describe('themeSnapshot', () => {
+  beforeEach(() => {
+    localStorage.clear();
+    document.documentElement.classList.remove('dark');
+  });
+
+  it('应从未知快照值回退到 system', () => {
+    localStorage.setItem(THEME_MODE_SNAPSHOT_KEY, JSON.stringify('invalid'));
+    expect(getThemeSyncSnapshot()).toBe('system');
+  });
+
+  it('resolveThemeMode 应解析 system 为当前系统主题', () => {
+    vi.spyOn(window, 'matchMedia').mockReturnValue({
+      matches: true,
+    } as MediaQueryList);
+
+    expect(resolveThemeMode('dark')).toBe('dark');
+    expect(resolveThemeMode('system')).toBe('dark');
+  });
+
+  it('applyThemeFromSnapshot 应根据快照同步 dark 类', () => {
+    persistThemeModeSnapshot('dark');
+    applyThemeFromSnapshot();
+    expect(document.documentElement.classList.contains('dark')).toBe(true);
+  });
+
+  it('applyResolvedThemeClass 移除 dark 时应清理 class', () => {
+    document.documentElement.classList.add('dark');
+    applyResolvedThemeClass('light');
+    expect(document.documentElement.classList.contains('dark')).toBe(false);
+  });
+});