feat: add CSS minification via lightningcss

- Cargo.toml: Add lightningcss 1.0.0-alpha.70 dep
- src/css.rs: New module with minify_css() function.
  Uses StyleSheet::parse() + minify() + to_css() pipeline.
  3 unit tests: whitespace removal, comment removal, selector merge.
- src/main.rs: Integrate minification into copy_static_assets().
  CSS files minified before writing; size delta logged.

Result: style.css 5670→4165 bytes (~27% smaller)

Cargo.toml

@@ -17,3 +17,6 @@ tree-sitter-bash      = "0.23"
 tree-sitter-highlight = "0.24"
 tree-sitter-json      = "0.24"
 tree-sitter-rust      = "0.23"
+
+# CSS processing
+lightningcss = "1.0.0-alpha.70"