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/ValidationModal.tsx

@@ -0,0 +1,33 @@
+import { h } from 'preact';
+import styles from './Modal.module.css';
+
+interface ValidationModalProps {
+  open: boolean;
+  message: string;
+  onClose: () => void;
+}
+
+/**
+ * Modal for displaying validation errors.
+ */
+const ValidationModal = ({ open, message, onClose }: ValidationModalProps) => {
+  if (!open) return null;
+  return (
+    <div className={styles['modal'] + ' ' + styles['show']} id="validation-modal" role="alertdialog" aria-modal="true" aria-labelledby="validation-modal-title">
+      <div className={styles['modal-content']}>
+        <div className={styles['modal-header']}>
+          <span className={styles['modal-title']} id="validation-modal-title">Fehler</span>
+          <button className={styles['close-button']} onClick={onClose} aria-label="Schließen">×</button>
+        </div>
+        <div className={styles['modal-body']}>
+          <div className={styles['modal-message']}>{message}</div>
+        </div>
+        <div className={styles['modal-footer']}>
+          <button className={styles['modal-button'] + ' ' + styles['cancel']} onClick={onClose} aria-label="OK">OK</button>
+        </div>
+      </div>
+    </div>
+  );
+};
+
+export default ValidationModal;