feat: NanumGothic font + SongSelect UI fixes

- Add NanumGothic.ttf and NanumGothic SDF.asset for Korean text support
- SongSelectManager: cardFont SerializeField for card title/artist rendering
- SongDetailPanel: difficulty selected color changed from blue to green

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Assets/Script/SongDetailPanel.cs

@@ -33,7 +33,7 @@ public class SongDetailPanel : MonoBehaviour
     [Header("씬 이름")]
     [SerializeField] private string gameSceneName = "Game";
 
-    private static readonly Color SelectedColor   = new Color(0.4f, 0.8f, 1f);
+    private static readonly Color SelectedColor   = new Color(0.2f, 0.78f, 0.4f);
     private static readonly Color DeselectedColor = Color.white;
 
     private SongInfo          currentSong;