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/hooks/useModal.ts

@@ -1,6 +1,6 @@
 import { useState, useCallback } from 'preact/hooks';
-import type { ModalState, ValidationState, CompletionModalState } from '../types/ui';
-import type { Game } from '../types/game';
+import type { ModalState, ValidationState, CompletionModalState } from '@lib/ui/types';
+import type { Game } from '@lib/domain/types';
 
 export function useModal() {
   const [modal, setModal] = useState<ModalState>({ open: false, gameId: null });