perf: reduce bundle size by using custom highlight.js subset

Replace highlight.js/lib/common (36 languages) with a curated subset
of 16 languages relevant for coding-assistant chat UIs. Both ToolCall
(direct hljs) and ChatMessage (rehype-highlight) now share the same
custom bundle from src/lib/highlight.ts.

Languages included: bash, css, diff, dockerfile, go, ini, javascript,
json, markdown, python, rust, shell, sql, typescript, xml, yaml.

Markdown chunk: 477KB → 336KB (-30%, -45KB gzipped)

src/components/ToolCall.tsx

@@ -1,6 +1,6 @@
 import { useState, useCallback, useMemo, useEffect, useRef } from 'react';
 import { ChevronRight, ChevronDown, Check, Copy, WrapText, AlignLeft } from 'lucide-react';
-import hljs from 'highlight.js/lib/common';
+import hljs from '../lib/highlight';
 import { useT } from '../hooks/useLocale';
 import { ImageBlock } from './ImageBlock';
 import { useToolCollapse } from '../hooks/useToolCollapse';