feat(frontend): M9 模态 UI（mode 系统+共享 store+WS 客户端 + chat(思维链/工具回显/情绪徽标)/code/Agent 监控/后台思考页 + Miuix 设计 token + PWA）；tsc+vite build 通过

frontend/web/src/lib/theme.ts

@@ -0,0 +1,22 @@
+export type ThemeMode = "dark" | "light";
+
+const THEME_KEY = "cyrene.theme";
+
+/** Read the persisted theme, defaulting to dark-first (Miuix). */
+export function readTheme(): ThemeMode {
+  const saved = localStorage.getItem(THEME_KEY);
+  return saved === "light" ? "light" : "dark";
+}
+
+/** Apply a theme by toggling the `.theme-light` class on <html>. */
+export function applyTheme(theme: ThemeMode): void {
+  document.documentElement.classList.toggle("theme-light", theme === "light");
+  localStorage.setItem(THEME_KEY, theme);
+  const meta = document.querySelector('meta[name="theme-color"]');
+  if (meta) meta.setAttribute("content", theme === "light" ? "#f6f6f8" : "#161616");
+}
+
+/** Apply the persisted theme on app start. */
+export function initTheme(): void {
+  applyTheme(readTheme());
+}