feat: implement 4-theme system with Zustand persistence

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

apps/web/components/providers/theme-provider.tsx

@@ -0,0 +1,26 @@
+'use client';
+
+import { useEffect } from 'react';
+import { useThemeStore } from '@/stores/theme-store';
+import { themes } from '@/themes';
+
+export function ThemeProvider({ children }: { children: React.ReactNode }) {
+  const { theme } = useThemeStore();
+
+  useEffect(() => {
+    const selectedTheme = themes[theme];
+    const root = document.documentElement;
+
+    Object.entries(selectedTheme.cssVars).forEach(([key, value]) => {
+      root.style.setProperty(key, value);
+    });
+
+    if (theme === 'dark') {
+      root.classList.add('dark');
+    } else {
+      root.classList.remove('dark');
+    }
+  }, [theme]);
+
+  return <>{children}</>;
+}