feat(01-02): design token system, i18n framework, and theme provider setup

- OKLCH design tokens with yellow #ffed00 primary and dark gray-blue dark mode
- next-intl cookie-based locale with DE/EN message files
- next-themes provider with system/light/dark support
- Sidebar and header layout dimension tokens
- Root layout with ThemeProvider and NextIntlClientProvider wrappers

apps/web/next.config.ts

@@ -1,7 +1,10 @@
 import type { NextConfig } from 'next';
+import createNextIntlPlugin from 'next-intl/plugin';
+
+const withNextIntl = createNextIntlPlugin('./src/i18n/request.ts');
 
 const nextConfig: NextConfig = {
   output: 'standalone' as const,
 };
 
-export default nextConfig;
+export default withNextIntl(nextConfig);