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/templates.rs

@@ -1,7 +1,7 @@
 //! HTML templates using maud.
 
 use crate::content::{Content, Frontmatter};
-use maud::{DOCTYPE, Markup, PreEscaped, html};
+use maud::{html, Markup, PreEscaped, DOCTYPE};
 
 /// Render a blog post with the base layout.
 pub fn render_post(frontmatter: &Frontmatter, content_html: &str) -> Markup {