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/tailwind.config.js

@@ -14,6 +14,34 @@ export default {
       },
     },
     extend: {
+      fontFamily: {
+        sans: [
+          "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",
+          '"Apple Color Emoji"',
+          '"Segoe UI Emoji"',
+        ],
+        mono: [
+          '"JetBrains Mono"',
+          '"Fira Code"',
+          '"Cascadia Code"',
+          '"Source Code Pro"',
+          "Menlo",
+          "Consolas",
+          "monospace",
+        ],
+      },
       borderRadius: {
         lg: "var(--radius)",
         md: "calc(var(--radius) - 2px)",