feat: integrate syntax highlighting into render pipeline

- src/render.rs: Rewrite with event-based pulldown-cmark parsing.
  Intercepts CodeBlock events and applies tree-sitter highlighting.
  4 new tests covering highlighted and unhighlighted code blocks.

- src/highlight.rs: Fix HtmlRenderer callback to return attributes
  only (not full span tags). Static HTML_ATTRS array for zero-alloc.

- static/style.css: Add 20 hl-* CSS classes with CSS variables.
  Light and dark mode via prefers-color-scheme.

Supported languages: rust, bash, json. Phase 2 complete.

flake.nix

@@ -42,6 +42,7 @@
               toolchain
               pkgs.treefmt
               pkgs.shfmt
+              pkgs.rust-analyzer
               pkgs.taplo
               pkgs.pkg-config
               pkgs.nixfmt