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/style.css.map

@@ -0,0 +1 @@
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