feat: add minimal base CSS with dark mode support

- static/style.css: CSS variable-based theming with:
  - Typography (system fonts, monospace for code)
  - Layout (centered content, nav/main/footer structure)
  - Dark mode via prefers-color-scheme
  - Component styles (posts, cards, tags, hero)

- src/main.rs: Add copy_static_assets() to copy static/
  directory to public/ during build

Phase 1 complete. Ready for syntax highlighting.

src/content.rs

@@ -1,7 +1,7 @@
 //! Content discovery and frontmatter parsing.
 
 use crate::error::{Error, Result};
-use gray_matter::{Matter, engine::YAML};
+use gray_matter::{engine::YAML, Matter};
 use std::fs;
 use std::path::{Path, PathBuf};