feat(tui): add syntax highlighting for code blocks

Add syntax highlighting to markdown code blocks in the TUI using
cli-highlight with a VS Code Dark-inspired color theme.

Features:
- 191 languages supported via highlight.js
- Auto-detection fallback for unknown languages
- Graceful fallback to plain styling on errors
- VS Code Dark-inspired color palette

Colors:
- Purple: keywords (const, function, if, etc.)
- Teal: built-ins (console, Math, print, etc.)
- Orange: strings
- Green: numbers, comments
- Yellow: function names
- Blue: literals (true, false, null)
- Red: diff deletions
- Light blue: variables, parameters

🤖 AI-assisted (Claude) - fully tested locally

src/tui/theme/theme.test.ts

@@ -0,0 +1,95 @@
+import { describe, expect, it } from "vitest";
+import { markdownTheme } from "./theme.js";
+
+describe("markdownTheme", () => {
+  describe("highlightCode", () => {
+    it("should highlight JavaScript code", () => {
+      const code = `const x = 42;`;
+      const result = markdownTheme.highlightCode!(code, "javascript");
+
+      expect(result).toBeInstanceOf(Array);
+      expect(result).toHaveLength(1);
+      // Should contain ANSI escape codes
+      expect(result[0]).toContain("\x1b[");
+    });
+
+    it("should highlight TypeScript code with multiple lines", () => {
+      const code = `function greet(name: string) {
+  return "Hello, " + name;
+}`;
+      const result = markdownTheme.highlightCode!(code, "typescript");
+
+      expect(result).toHaveLength(3);
+      // Each line should have highlighting
+      result.forEach((line) => {
+        expect(line).toContain("\x1b[");
+      });
+    });
+
+    it("should highlight Python code", () => {
+      const code = `def hello():
+    print("world")`;
+      const result = markdownTheme.highlightCode!(code, "python");
+
+      expect(result).toHaveLength(2);
+      expect(result[0]).toContain("\x1b["); // def keyword colored
+    });
+
+    it("should handle unknown languages with auto-detection", () => {
+      const code = `const x = 42;`;
+      const result = markdownTheme.highlightCode!(code, "not-a-real-language");
+
+      expect(result).toBeInstanceOf(Array);
+      expect(result).toHaveLength(1);
+      // Should still return something (auto-detected or fallback)
+      expect(result[0].length).toBeGreaterThan(0);
+    });
+
+    it("should handle code without language specifier", () => {
+      const code = `echo "hello"`;
+      const result = markdownTheme.highlightCode!(code, undefined);
+
+      expect(result).toBeInstanceOf(Array);
+      expect(result).toHaveLength(1);
+    });
+
+    it("should handle empty code", () => {
+      const result = markdownTheme.highlightCode!("", "javascript");
+
+      expect(result).toBeInstanceOf(Array);
+      expect(result).toHaveLength(1);
+      expect(result[0]).toBe("");
+    });
+
+    it("should highlight bash/shell code", () => {
+      const code = `#!/bin/bash
+echo "Hello"
+for i in {1..5}; do
+  echo $i
+done`;
+      const result = markdownTheme.highlightCode!(code, "bash");
+
+      expect(result).toHaveLength(5);
+      // Should have colored output
+      expect(result.some((line) => line.includes("\x1b["))).toBe(true);
+    });
+
+    it("should highlight JSON", () => {
+      const code = `{"name": "test", "count": 42, "active": true}`;
+      const result = markdownTheme.highlightCode!(code, "json");
+
+      expect(result).toHaveLength(1);
+      expect(result[0]).toContain("\x1b[");
+    });
+
+    it("should handle code with special characters", () => {
+      const code = `const regex = /\\d+/g;
+const str = "Hello\\nWorld";`;
+      const result = markdownTheme.highlightCode!(code, "javascript");
+
+      expect(result).toHaveLength(2);
+      // Should not throw and should return valid output
+      expect(result[0].length).toBeGreaterThan(0);
+    });
+  });
+});