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/src/i18n/request.ts

@@ -0,0 +1,12 @@
+import { getRequestConfig } from 'next-intl/server';
+import { cookies } from 'next/headers';
+
+export default getRequestConfig(async () => {
+  const cookieStore = await cookies();
+  const locale = cookieStore.get('NEXT_LOCALE')?.value || 'de';
+
+  return {
+    locale,
+    messages: (await import(`../messages/${locale}.json`)).default,
+  };
+});