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/TextMesh Pro/Fonts/NanumGothic.ttf.meta

@@ -0,0 +1,21 @@
+fileFormatVersion: 2
+guid: d2abafb75e12afe48bb523e2ac30b43b
+TrueTypeFontImporter:
+  externalObjects: {}
+  serializedVersion: 4
+  fontSize: 16
+  forceTextureCase: -2
+  characterSpacing: 0
+  characterPadding: 1
+  includeFontData: 1
+  fontNames:
+  - NanumGothic
+  fallbackFontReferences: []
+  customCharacters: 
+  fontRenderingMode: 0
+  ascentCalculationMode: 1
+  useLegacyBoundsCalculation: 0
+  shouldRoundAdvanceValue: 1
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: