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)

src/css.rs

@@ -0,0 +1,80 @@
+//! CSS processing via lightningcss.
+
+use lightningcss::stylesheet::{MinifyOptions, ParserOptions, PrinterOptions, StyleSheet};
+
+/// Minify CSS content.
+///
+/// Returns minified CSS string on success, or the original input on error.
+pub fn minify_css(css: &str) -> String {
+    match try_minify(css) {
+        Ok(minified) => minified,
+        Err(_) => css.to_string(),
+    }
+}
+
+fn try_minify(css: &str) -> Result<String, Box<dyn std::error::Error>> {
+    let mut stylesheet = StyleSheet::parse(css, ParserOptions::default())
+        .map_err(|e| format!("parse error: {:?}", e))?;
+
+    stylesheet
+        .minify(MinifyOptions::default())
+        .map_err(|e| format!("minify error: {:?}", e))?;
+
+    let result = stylesheet
+        .to_css(PrinterOptions {
+            minify: true,
+            ..Default::default()
+        })
+        .map_err(|e| format!("print error: {:?}", e))?;
+
+    Ok(result.code)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_minify_removes_whitespace() {
+        let input = r#"
+            .foo {
+                color: red;
+            }
+        "#;
+        let output = minify_css(input);
+
+        // Should be smaller (whitespace removed)
+        assert!(output.len() < input.len());
+        // Should still contain the essential content
+        assert!(output.contains(".foo"));
+        assert!(output.contains("color"));
+        assert!(output.contains("red"));
+    }
+
+    #[test]
+    fn test_minify_removes_comments() {
+        let input = r#"
+            /* This is a comment */
+            .bar { background: blue; }
+        "#;
+        let output = minify_css(input);
+
+        // Comment should be removed
+        assert!(!output.contains("This is a comment"));
+        // Rule should remain
+        assert!(output.contains(".bar"));
+    }
+
+    #[test]
+    fn test_minify_merges_selectors() {
+        let input = r#"
+            .foo { color: red; }
+            .bar { color: red; }
+        "#;
+        let output = minify_css(input);
+
+        // Should merge identical rules
+        // Either ".foo,.bar" or ".bar,.foo" pattern
+        assert!(output.contains(","));
+    }
+}