chat markdown polish: bold headings, multi-hue syntax theme

The first hljs theme mapped every token class to orange shades —
technically highlighted, visually monochrome. Replaced with a CRT
phosphor palette (orange keywords, green strings, amber numbers, cyan
function names, purple built-ins) over a neutral base inside code
blocks. Headings get bold weight and top margin; paragraphs and list
items get relaxed leading.

frontend/app/globals.css

@@ -160,7 +160,13 @@ body {
   background: var(--accent);
 }
 
-/* Syntax highlighting (rehype-highlight) — retro palette, no stock theme */
+/* Syntax highlighting (rehype-highlight) — CRT phosphor palette:
+   distinct hues per token class, not just orange shades */
+.md-body pre code {
+  color: #d6d3d1; /* neutral base inside blocks; tokens colorize below */
+  background: transparent;
+  padding: 0;
+}
 .hljs-comment,
 .hljs-quote {
   color: #78716c;
@@ -168,30 +174,30 @@ body {
 }
 .hljs-keyword,
 .hljs-selector-tag,
-.hljs-literal,
 .hljs-doctag {
-  color: #fb923c;
+  color: #fb923c; /* orange — the signature accent */
 }
 .hljs-string,
 .hljs-regexp,
 .hljs-addition {
-  color: #fbbf24;
+  color: #4ade80; /* phosphor green */
 }
 .hljs-number,
 .hljs-symbol,
-.hljs-bullet {
-  color: #fdba74;
+.hljs-bullet,
+.hljs-literal {
+  color: #facc15; /* amber terminal yellow */
 }
 .hljs-title,
 .hljs-title.function_,
 .hljs-title.class_,
 .hljs-section {
-  color: #fed7aa;
+  color: #22d3ee; /* cyan */
 }
 .hljs-built_in,
 .hljs-type,
 .hljs-class {
-  color: #f59e0b;
+  color: #c084fc; /* magenta-purple */
 }
 .hljs-attr,
 .hljs-attribute,

frontend/components/Markdown.tsx

@@ -25,7 +25,7 @@ export function Markdown({
   highlight?: boolean;
 }) {
   return (
-    <div className="md-body space-y-2 [&_a]:text-orange-400 [&_a]:underline [&_blockquote]:border-l-4 [&_blockquote]:border-stone-700 [&_blockquote]:pl-3 [&_blockquote]:text-stone-400 [&_code]:bg-black [&_code]:px-1 [&_code]:text-orange-300 [&_h1]:text-xl [&_h1]:text-orange-400 [&_h2]:text-lg [&_h2]:text-orange-400 [&_h3]:text-orange-300 [&_hr]:border-stone-700 [&_li]:ml-5 [&_ol]:list-decimal [&_pre]:overflow-x-auto [&_pre]:border-2 [&_pre]:border-stone-700 [&_pre]:bg-black [&_pre]:p-2 [&_strong]:text-orange-200 [&_table]:border-2 [&_table]:border-stone-700 [&_td]:border [&_td]:border-stone-700 [&_td]:px-2 [&_th]:border [&_th]:border-stone-600 [&_th]:bg-stone-900 [&_th]:px-2 [&_ul]:list-disc">
+    <div className="md-body space-y-2 [&_a]:text-orange-400 [&_a]:underline [&_blockquote]:border-l-4 [&_blockquote]:border-stone-700 [&_blockquote]:pl-3 [&_blockquote]:text-stone-400 [&_code]:bg-black [&_code]:px-1 [&_code]:text-orange-300 [&_h1]:mt-3 [&_h1]:text-xl [&_h1]:font-bold [&_h1]:text-orange-400 [&_h2]:mt-3 [&_h2]:text-lg [&_h2]:font-bold [&_h2]:text-orange-400 [&_h3]:mt-2 [&_h3]:font-semibold [&_h3]:text-orange-300 [&_h4]:font-semibold [&_h4]:text-orange-200 [&_hr]:border-stone-700 [&_li]:ml-5 [&_li]:leading-relaxed [&_ol]:list-decimal [&_p]:leading-relaxed [&_pre]:overflow-x-auto [&_pre]:border-2 [&_pre]:border-stone-700 [&_pre]:bg-black [&_pre]:p-3 [&_strong]:font-bold [&_strong]:text-orange-200 [&_table]:border-2 [&_table]:border-stone-700 [&_td]:border [&_td]:border-stone-700 [&_td]:px-2 [&_th]:border [&_th]:border-stone-600 [&_th]:bg-stone-900 [&_th]:px-2 [&_ul]:list-disc">
       <ReactMarkdown
         remarkPlugins={[remarkGfm, remarkMath]}
         rehypePlugins={