feat: add TOML site config for metadata

- Cargo.toml: Add toml and serde dependencies
- site.toml: New config with title, author, base_url
- src/config.rs: SiteConfig struct with load() function
- src/error.rs: Add Error::Config for parse errors
- src/main.rs: Load site.toml, thread config and page paths
  through generators
- src/templates.rs: Use config.title in nav/titles,
  config.author in footer, config.base_url for canonical URLs.

All three config fields verified in generated HTML output.

Cargo.toml

@@ -20,3 +20,7 @@ tree-sitter-rust      = "0.23"
 
 # CSS processing
 lightningcss = "1.0.0-alpha.70"
+
+# Config parsing
+serde = { version = "1", features = ["derive"] }
+toml  = "0.8"