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.

src/error.rs

@@ -36,6 +36,10 @@ pub enum Error {
     /// Content directory not found.
     #[error("content directory not found: {0}")]
     ContentDirNotFound(PathBuf),
+
+    /// Failed to parse configuration file.
+    #[error("invalid config in {path}: {message}")]
+    Config { path: PathBuf, message: String },
 }
 
 /// Result type alias for compiler operations.