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/SongSelectManager.cs

@@ -14,6 +14,7 @@ public class SongSelectManager : MonoBehaviour
     [SerializeField] private GameObject      loadingOverlay;
     [SerializeField] private GameObject      errorOverlay;
     [SerializeField] private TMP_Text        errorText;
+    [SerializeField] private TMP_FontAsset   cardFont;
 
     private static readonly Color TabActive   = Color.white;
     private static readonly Color TabInactive = new Color(0.6f, 0.6f, 0.6f);
@@ -129,6 +130,7 @@ public class SongSelectManager : MonoBehaviour
         tr.offsetMin = new Vector2(5f, 0f);
         tr.offsetMax = new Vector2(downloaded ? -18f : -3f, -1f);
         var tTmp = titleGO.AddComponent<TextMeshProUGUI>();
+        if (cardFont != null) tTmp.font = cardFont;
         tTmp.text         = song.title;
         tTmp.fontSize     = 5f;
         tTmp.color        = Color.white;
@@ -144,6 +146,7 @@ public class SongSelectManager : MonoBehaviour
         ar.offsetMin = new Vector2(5f, 1f);
         ar.offsetMax = new Vector2(-3f, 0f);
         var aTmp = artistGO.AddComponent<TextMeshProUGUI>();
+        if (cardFont != null) aTmp.font = cardFont;
         aTmp.text      = song.artist;
         aTmp.fontSize  = 4f;
         aTmp.color     = new Color(1f, 1f, 1f, 0.6f);