feat(tui): per-language syntax highlighting in markdown code fences

Adds a minimal hand-rolled highlighter for ts/js/jsx/tsx, py, sh/bash, go, rust,
json, yaml, sql. Recognizes whole-line comments, single/double/backtick strings,
numbers, and per-language keyword sets. Unknown langs fall through to the current
plain rendering; the existing diff-specific colorization is preserved.

Closes the §8 "Markdown syntax highlighting is missing (only diff gets colored)"
finding from the TUI v2 audit without pulling in a highlighter library.

ui-tui/src/__tests__/syntax.test.ts

@@ -0,0 +1,45 @@
+import { describe, expect, it } from 'vitest'
+
+import { highlightLine, isHighlightable } from '../lib/syntax.js'
+import { DEFAULT_THEME } from '../theme.js'
+
+const t = DEFAULT_THEME
+
+describe('syntax highlighter', () => {
+  it('recognizes supported langs and aliases', () => {
+    expect(isHighlightable('ts')).toBe(true)
+    expect(isHighlightable('js')).toBe(true)
+    expect(isHighlightable('python')).toBe(true)
+    expect(isHighlightable('rs')).toBe(true)
+    expect(isHighlightable('bash')).toBe(true)
+    expect(isHighlightable('whatever')).toBe(false)
+    expect(isHighlightable('')).toBe(false)
+  })
+
+  it('paints a whole-line comment dim', () => {
+    const tokens = highlightLine('// hello', 'ts', t)
+
+    expect(tokens).toEqual([[t.color.dim, '// hello']])
+  })
+
+  it('paints keywords, strings, and numbers in a ts line', () => {
+    const tokens = highlightLine(`const x = 'hi' + 42`, 'ts', t)
+    const colors = tokens.map(tok => tok[0])
+
+    expect(colors).toContain(t.color.bronze) // const
+    expect(colors).toContain(t.color.amber) // 'hi'
+    expect(colors).toContain(t.color.cornsilk) // 42
+  })
+
+  it('falls through unchanged for unknown langs', () => {
+    const tokens = highlightLine(`const x = 1`, 'zzz', t)
+
+    expect(tokens).toEqual([['', 'const x = 1']])
+  })
+
+  it('treats `#` as a python comment, not a selector', () => {
+    const tokens = highlightLine('# comment', 'py', t)
+
+    expect(tokens).toEqual([[t.color.dim, '# comment']])
+  })
+})