feat(theme): Zustand 主题 store，localStorage 持久化，默认浅色

frontend/src/stores/theme.ts

@@ -0,0 +1,22 @@
+import { create } from 'zustand'
+import { persist } from 'zustand/middleware'
+
+type Theme = 'light' | 'dark'
+
+interface ThemeState {
+  theme: Theme
+  toggleTheme: () => void
+  setTheme: (t: Theme) => void
+}
+
+export const useThemeStore = create<ThemeState>()(
+  persist(
+    (set) => ({
+      theme: 'light',
+      toggleTheme: () =>
+        set((state) => ({ theme: state.theme === 'light' ? 'dark' : 'light' })),
+      setTheme: (t) => set({ theme: t }),
+    }),
+    { name: 'gen2d_theme' },
+  ),
+)