- Add explicit CJK fonts (PingFang SC, Noto Sans SC, Microsoft YaHei) and
programmer fonts (JetBrains Mono, Fira Code, Cascadia Code) to Tailwind config
- Bump prose base size from prose-sm (14px) to prose-lg (18px) for sharper CJK rendering
- Unify user/assistant message font size at 18px with CJK-aware line-height (1.8)
- Replace pure black/white foreground with Apple-style warm grays (#1d1d1f / #f5f5f7)
- Override Tailwind Typography colors to use design tokens for consistency
- Add negative letter-spacing on headings for tighter, more polished look
