feat: Refactor project structure and add new files

- Removed old style.css and added boilerplate.css for better cross-browser compatibility.
- Introduced browserconfig.xml for Microsoft tile support.
- Created impressum.php and index.php for main application pages.
- Added info.php for PHP info display.
- Implemented mett.js and mett.min.js for Mett calculation functionality.
- Included respond.min.js for media query support in older browsers.
- Recreated style.css with responsive design features.
- Added index.html as a landing page with a placeholder message.

dev/browserconfig.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8"?>
+<browserconfig>
+    <msapplication>
+        <tile>
+            <square150x150logo src="/mstile-150x150.png"/>
+            <TileColor>#b91d47</TileColor>
+        </tile>
+    </msapplication>
+</browserconfig>