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/new-game/BreakRuleStep.tsx

@@ -1,7 +1,7 @@
 import { h } from 'preact';
 import { useState } from 'preact/hooks';
 import styles from '../NewGame.module.css';
-import type { BreakRule } from '../../types/game';
+import type { BreakRule } from '@lib/domain/types';
 
 interface BreakRuleStepProps {
   onNext: (rule: BreakRule) => void;