feat: add tree-sitter syntax highlighting module

- Cargo.toml: Add tree-sitter-highlight + grammar crates
  (rust, bash, json). TOML dropped due to API incompatibility.
- src/highlight.rs: Language enum, highlight_code() function,
  4 unit tests covering parsing and HTML generation.
- Uses static HTML_ATTRS array for zero-allocation rendering.

src/main.rs

@@ -4,6 +4,7 @@
 
 mod content;
 mod error;
+mod highlight;
 mod render;
 mod templates;