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;