feat: add optimized system font stack for Korean and English (#15)

Add native system fonts optimized for each platform to improve
typography and rendering performance:
- macOS/iOS: Apple SD Gothic Neo
- Windows: Malgun Gothic / Segoe UI
- Android: Roboto

This provides instant rendering with zero network requests
and platform-native look and feel for both Korean and English content.

Author: amondnet

apps/web/app/assets/css/main.css

@@ -1,2 +1,8 @@
 @import "tailwindcss";
 @import "@nuxt/ui";
+
+@theme {
+  /* 한글 + 영문 시스템 폰트 스택 */
+  --font-sans: -apple-system, BlinkMacSystemFont, 'Apple SD Gothic Neo',
+               'Malgun Gothic', 'Segoe UI', Roboto, sans-serif;
+}