refactor: migrate UI to Preact components and remove legacy Astro/JS

- Replaced all .astro components with .jsx Preact components and added corresponding CSS modules.
- Updated index.astro to use the new App Preact component; removed legacy script and Astro imports.
- Deleted obsolete .astro component files and main JS logic (src/scripts/index.js, public/scripts/index.js).
- Updated astro.config.mjs for Preact integration.
- Updated package.json and package-lock.json to include @astrojs/preact and preact.
- Updated tsconfig.json for Preact JSX support.
- Refactored index.css to keep only global resets and utility styles.
- All changes relate to Gitea issue #1 (refactor to astro app).

Migrates the UI from Astro/vanilla JS to a modular Preact component architecture, removing all legacy code and aligning the project with modern best practices.
Refs #1

src/components/ValidationModal.jsx

@@ -0,0 +1,22 @@
+import { h } from 'preact';
+import styles from './ValidationModal.module.css';
+
+export default function ValidationModal({ open, message, onClose }) {
+  if (!open) return null;
+  return (
+    <div className={styles['modal'] + ' ' + styles['show']} id="validation-modal">
+      <div className={styles['modal-content']}>
+        <div className={styles['modal-header']}>
+          <span className={styles['modal-title']}>Fehler</span>
+          <button className={styles['close-button']} onClick={onClose}>×</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}>OK</button>
+        </div>
+      </div>
+    </div>
+  );
+}