feat: implement 4-theme system with Zustand persistence

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/stores/theme-store.ts

@@ -0,0 +1,20 @@
+import { create } from 'zustand';
+import { persist } from 'zustand/middleware';
+import type { ThemeName } from '@/themes';
+
+interface ThemeState {
+  theme: ThemeName;
+  setTheme: (theme: ThemeName) => void;
+}
+
+export const useThemeStore = create<ThemeState>()(
+  persist(
+    (set) => ({
+      theme: 'light',
+      setTheme: (theme) => set({ theme }),
+    }),
+    {
+      name: 'horux-theme',
+    }
+  )
+);