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/lib/ui/Layout.tsx

@@ -0,0 +1,33 @@
+import { h } from 'preact';
+import styles from './Layout.module.css';
+
+interface LayoutProps {
+  children: any;
+  className?: string;
+}
+
+export function Layout({ children, className = '' }: LayoutProps) {
+  return (
+    <div className={`${styles.layout} ${className}`}>
+      <a href="#main-content" className="skip-link">
+        Zum Hauptinhalt springen
+      </a>
+      <div className={styles.content} id="main-content">
+        {children}
+      </div>
+    </div>
+  );
+}
+
+interface ScreenProps {
+  children: any;
+  className?: string;
+}
+
+export function Screen({ children, className = '' }: ScreenProps) {
+  return (
+    <div className={`${styles.screen} ${className}`}>
+      {children}
+    </div>
+  );
+}