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 SDF.asset.meta

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Assets/TextMesh Pro/Fonts/NanumGothic.ttf.meta

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