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/src/components/MarkdownTextRenderer.tsx

@@ -24,10 +24,10 @@ export default function MarkdownTextRenderer({
   return (
     <div
       className={cn(
-        "markdown-content prose prose-sm max-w-none dark:prose-invert",
-        "prose-headings:mt-4 prose-headings:mb-2 prose-headings:font-semibold",
-        "prose-h1:text-lg prose-h2:text-base prose-h3:text-[0.95rem] prose-h4:text-sm",
-        "prose-p:my-2 prose-p:leading-relaxed",
+        "markdown-content prose prose-lg max-w-none dark:prose-invert",
+        "prose-headings:mt-4 prose-headings:mb-2 prose-headings:font-semibold prose-headings:tracking-tight",
+        "prose-h1:text-xl prose-h2:text-lg prose-h3:text-base prose-h4:text-sm",
+        "prose-p:my-2",
         "prose-ul:my-2 prose-ol:my-2 prose-li:my-0.5",
         "prose-blockquote:my-3 prose-blockquote:border-l-2 prose-blockquote:font-normal",
         "prose-blockquote:not-italic prose-blockquote:text-foreground/80",
@@ -38,6 +38,7 @@ export default function MarkdownTextRenderer({
         "prose-table:my-3 prose-th:text-left prose-th:font-medium",
         className,
       )}
+      style={{ lineHeight: "var(--cjk-line-height)" }}
     >
       <ReactMarkdown
         remarkPlugins={[remarkGfm, remarkMath]}