feat(markdown): add /g/ [code] tag syntax highlighting

Parse [code] blocks on /g/ into Prettify-style highlighted code boxes while leaving the tags as literal text on other boards.

src/lib/utils/__tests__/code-highlight.test.ts

@@ -0,0 +1,62 @@
+import { describe, it, expect } from 'vitest';
+import { highlightCode, type CodeToken } from '../code-highlight';
+
+// Adjacent plain tokens (including whitespace) are merged, like Prettify, so match by trimmed value.
+const classOf = (tokens: CodeToken[], value: string): string | undefined => tokens.find((token) => token.value.trim() === value)?.cls;
+
+const reassemble = (tokens: CodeToken[]): string => tokens.map((token) => token.value).join('');
+
+describe('highlightCode', () => {
+  it('preserves the source exactly when tokens are concatenated', () => {
+    const source = '.element {\n  animation-delay: calc((sibling-index() - 1) * 100ms);\n}';
+    expect(reassemble(highlightCode(source))).toBe(source);
+  });
+
+  it('classifies the 4chan CSS example like Prettify (punctuation, plain, literals)', () => {
+    const tokens = highlightCode('.element {\n  animation-delay: calc((sibling-index() - 1) * 100ms);\n}');
+    // Numbers (incl. CSS units) are literals.
+    expect(classOf(tokens, '1')).toBe('lit');
+    expect(classOf(tokens, '100ms')).toBe('lit');
+    // Hyphenated CSS identifiers split on punctuation, exactly as Prettify renders them.
+    expect(classOf(tokens, 'animation')).toBe('pln');
+    expect(classOf(tokens, 'delay')).toBe('pln');
+    expect(tokens.some((token) => token.cls === 'pun' && token.value.includes('{'))).toBe(true);
+  });
+
+  it('highlights keywords from the union of languages (like Prettify with no language)', () => {
+    const tokens = highlightCode('select string body');
+    expect(classOf(tokens, 'select')).toBe('kwd');
+    expect(classOf(tokens, 'string')).toBe('kwd');
+    expect(classOf(tokens, 'body')).toBe('pln');
+  });
+
+  it('highlights strings, including unterminated ones', () => {
+    expect(classOf(highlightCode('font-family: "Consolas";'), '"Consolas"')).toBe('str');
+    expect(classOf(highlightCode("x = 'hi'"), "'hi'")).toBe('str');
+    expect(highlightCode('s = "unterminated').some((token) => token.cls === 'str' && token.value === '"unterminated')).toBe(true);
+  });
+
+  it('highlights line, block, and hash comments', () => {
+    expect(classOf(highlightCode('x = 1 // note'), '// note')).toBe('com');
+    expect(classOf(highlightCode('a /* mid */ b'), '/* mid */')).toBe('com');
+    expect(classOf(highlightCode('x = 1 # python'), '# python')).toBe('com');
+  });
+
+  it('does not treat CSS hex colors as comments', () => {
+    const tokens = highlightCode('color: #fff;');
+    expect(tokens.some((token) => token.cls === 'com')).toBe(false);
+    expect(classOf(tokens, '#')).toBe('pun');
+    expect(classOf(tokens, 'fff')).toBe('pln');
+  });
+
+  it('treats keywords individually and merges adjacent plain runs', () => {
+    const tokens = highlightCode('const x');
+    expect(classOf(tokens, 'const')).toBe('kwd');
+    // " x" (leading space + identifier) merges into a single plain token.
+    expect(tokens.find((token) => token.value === ' x')?.cls).toBe('pln');
+  });
+
+  it('returns no tokens for empty input', () => {
+    expect(highlightCode('')).toEqual([]);
+  });
+});