refactor: extract reusable library

- move reusable domain, data, state, ui code into src/lib
- update host screens to consume new library exports
- document architecture and configure path aliases
- bump astro integration dependencies for compatibility

Refs #30

src/components/GameCompletionModal.tsx

@@ -1,7 +1,7 @@
 import { h } from 'preact';
-import modalStyles from './Modal.module.css';
+import modalStyles from '@lib/ui/Modal.module.css';
 import styles from './GameCompletionModal.module.css';
-import type { Game } from '../types/game';
+import type { Game } from '@lib/domain/types';
 
 interface GameCompletionModalProps {
   open: boolean;