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/themeSnapshot.ts

@@ -0,0 +1,50 @@
+export type ThemeMode = 'light' | 'dark' | 'system';
+export type ResolvedThemeMode = 'light' | 'dark';
+
+export const THEME_MODE_STORAGE_KEY = 'app/themeMode' as const;
+export const THEME_MODE_SNAPSHOT_KEY = 'snapshot/app/themeMode';
+
+const VALID_MODES: ThemeMode[] = ['light', 'dark', 'system'];
+
+export const isValidThemeMode = (value: unknown): value is ThemeMode =>
+  VALID_MODES.includes(value as ThemeMode);
+
+/**
+ * 同步读取 localStorage 快照（首屏防闪烁）
+ */
+export const getThemeSyncSnapshot = (): ThemeMode => {
+  try {
+    const raw = localStorage.getItem(THEME_MODE_SNAPSHOT_KEY);
+    if (!raw) return 'system';
+    const parsed = JSON.parse(raw) as unknown;
+    return isValidThemeMode(parsed) ? parsed : 'system';
+  } catch {
+    return 'system';
+  }
+};
+
+export const getSystemThemeMode = (): ResolvedThemeMode => {
+  if (typeof window === 'undefined') return 'light';
+  return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
+};
+
+export const resolveThemeMode = (mode: ThemeMode): ResolvedThemeMode =>
+  mode === 'system' ? getSystemThemeMode() : mode;
+
+export const persistThemeModeSnapshot = (mode: ThemeMode): void => {
+  try {
+    localStorage.setItem(THEME_MODE_SNAPSHOT_KEY, JSON.stringify(mode));
+  } catch (error) {
+    console.error('[Theme Snapshot Error] LocalStorage quota exceeded:', error);
+  }
+};
+
+export const applyResolvedThemeClass = (resolved: ResolvedThemeMode): void => {
+  if (typeof document === 'undefined') return;
+  document.documentElement.classList.toggle('dark', resolved === 'dark');
+};
+
+/** 在 React 挂载前同步应用快照主题，避免首帧闪烁 */
+export const applyThemeFromSnapshot = (): void => {
+  applyResolvedThemeClass(resolveThemeMode(getThemeSyncSnapshot()));
+};