feat(tui): implement syntax highlighting for code blocks

Add syntax.rs with syntect integration:
- SyntaxHighlighter with configurable themes
- Support for Rust, Python, JavaScript, TypeScript, etc.
- Language alias normalization (js→JavaScript, py→Python)
- highlight_code() and render_code_block() helpers
- Global syntax/theme sets with lazy loading
- Fallback for unknown languages
- 10 unit tests

Dependencies added: syntect, once_cell

Closes #12

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Cargo.toml

@@ -37,6 +37,7 @@ crossterm = { version = "0.29.0", features = ["bracketed-paste", "event-stream"]
 textwrap = "0.16"
 unicode-width = "0.2"
 pulldown-cmark = "0.12"
+syntect = "5"
 
 # CLI
 clap = { version = "4", features = ["derive"] }
@@ -59,3 +60,4 @@ reqwest = { version = "0.12", features = ["json", "stream"] }
 # Utilities
 chrono = { version = "0.4", features = ["serde"] }
 uuid = { version = "1", features = ["v4", "serde"] }
+once_cell = "1"