refactor(js): remove jQuery dependency and replace with vanilla JS

- Rewrite haus-schleusingen.js entirely in vanilla JavaScript
- Use IntersectionObserver instead of scroll event for scroll animations
- Replace jQuery slideUp/slideDown with display toggle for accordion
- Replace jQuery fadeIn with CSS opacity transition for form success
- Remove jQuery CDN script tag from haus-schleusingen.html
- Delete unused masonry.pkgd.min.js
- Remove jquery globals from eslint.config.js

Ref #19

eslint.config.js

@@ -13,7 +13,6 @@ module.exports = [
       sourceType: "script",
       globals: {
         ...globals.browser,
-        ...globals.jquery,
       },
     },
     plugins: {