feat(math): add katex-rs math rendering module

Add src/math.rs with render_math() wrapper around katex-rs for
build-time LaTeX to HTML conversion. Wire module into main.rs.

- Use KatexContext::default() for standard LaTeX function registry
- Settings: display_mode toggle, throw_on_error=false for graceful
  degradation
- Include unit tests for inline/display math rendering

src/main.rs

@@ -8,6 +8,7 @@ mod css;
 mod error;
 mod feed;
 mod highlight;
+mod math;
 mod render;
 mod templates;
 

src/math.rs

@@ -0,0 +1,48 @@
+//! Math rendering via katex-rs.
+//!
+//! Converts LaTeX math expressions to HTML at build-time.
+
+use katex::{render_to_string, KatexContext, Settings};
+
+/// Render a LaTeX math expression to HTML.
+///
+/// # Arguments
+/// * `latex` - The LaTeX source string
+/// * `display_mode` - `true` for block equations, `false` for inline
+///
+/// # Returns
+/// The rendered HTML string, or an error message on failure.
+pub fn render_math(latex: &str, display_mode: bool) -> Result<String, String> {
+    let ctx = KatexContext::default();
+    let settings = Settings::builder()
+        .display_mode(display_mode)
+        .throw_on_error(false)
+        .build();
+
+    render_to_string(&ctx, latex, &settings).map_err(|e| e.to_string())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_inline_math() {
+        let result = render_math("x^2", false).unwrap();
+        assert!(result.contains("<span"));
+    }
+
+    #[test]
+    fn test_display_math() {
+        let result = render_math(r"\sum_{i=1}^n i", true).unwrap();
+        assert!(result.contains("<span"));
+    }
+
+    #[test]
+    fn test_invalid_latex_no_panic() {
+        // Should not panic, returns error or graceful fallback
+        let result = render_math(r"\invalidcommand", false);
+        // katex-rs with throw_on_error=false returns error markup
+        assert!(result.is_ok() || result.is_err());
+    }
+}