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/config.rs

@@ -0,0 +1,51 @@
+//! Site configuration loading.
+
+use crate::error::{Error, Result};
+use serde::Deserialize;
+use std::fs;
+use std::path::Path;
+
+/// Site-wide configuration loaded from site.toml.
+#[derive(Debug, Deserialize)]
+pub struct SiteConfig {
+    /// Site title (used in page titles and nav).
+    pub title: String,
+    /// Site author name.
+    pub author: String,
+    /// Base URL for the site (used for feeds, canonical links).
+    pub base_url: String,
+}
+
+impl SiteConfig {
+    /// Load configuration from a TOML file.
+    pub fn load(path: &Path) -> Result<Self> {
+        let content = fs::read_to_string(path).map_err(|e| Error::ReadFile {
+            path: path.to_path_buf(),
+            source: e,
+        })?;
+
+        toml::from_str(&content).map_err(|e| Error::Config {
+            path: path.to_path_buf(),
+            message: e.to_string(),
+        })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_parse_config() {
+        let toml = r#"
+            title = "Test Site"
+            author = "Test Author"
+            base_url = "https://example.com/"
+        "#;
+
+        let config: SiteConfig = toml::from_str(toml).unwrap();
+        assert_eq!(config.title, "Test Site");
+        assert_eq!(config.author, "Test Author");
+        assert_eq!(config.base_url, "https://example.com/");
+    }
+}