style(webui): improve typography with Apple-inspired font stack and CJK support

- 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

webui/index.html

@@ -26,8 +26,10 @@
         background: #ffffff;
         color: #0a0a0a;
         font-family:
-          ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI",
-          Roboto, "Helvetica Neue", Arial, "Noto Sans", sans-serif;
+          system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto,
+          "Helvetica Neue", Arial, "Noto Sans", "Noto Sans SC",
+          "PingFang SC", "Hiragino Sans GB", "Microsoft YaHei",
+          sans-serif;
       }
 
       html.dark body {