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

astro.config.mjs

@@ -1,5 +1,9 @@
 // @ts-check
 import { defineConfig } from 'astro/config';
 
+import preact from '@astrojs/preact';
+
 // https://astro.build/config
-export default defineConfig({});
+export default defineConfig({
+  integrations: [preact()]
+});