refactor: migrate CSS to SCSS with compilation setup

- Move all CSS files to public/assets/css/src/ as SCSS
- Add public/assets/css/style.scss as main entry point
- Compile SCSS to public/assets/css/style.css
- Update src/style.css to import compiled CSS
- Add .css.map for source maps

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

public/assets/css/src/print-styles.scss

@@ -0,0 +1,17 @@
+/* PagedJS print styles */
+@page {
+  size: A4;
+  margin: 20mm 15mm 26mm 15mm;
+}
+h2 {
+  break-before: page;
+}
+
+@page {
+  @bottom-center {
+    content: string(title);
+  }
+}
+.chapter > h2 {
+  string-set: title content(text);
+}