fix: modal overlay and game screen styling

- Move modal overlay CSS to global stylesheet for reliable overlay behavior
- Render GameCompletionModal outside main container for true overlay effect
- Refactor GameCompletionModal to use global overlay and local content styles
- Fix player score layout, color, and button styling on game detail screen
- Add global .modal and .modal.show classes to index.css
- Clean up CSS modules for modal content and responsive design

Fixes #<issue_number>

src/components/GameCompletionModal.jsx

@@ -11,7 +11,7 @@ export default function GameCompletionModal({ open, game, onConfirm, onClose })
     ? `Unentschieden zwischen ${winners.join(' und ')}`
     : `${winners[0]} hat gewonnen!`;
   return (
-    <div className={styles['modal'] + ' ' + styles['show']} id="game-completion-modal">
+    <div id="game-completion-modal" className="modal show">
       <div className={styles['modal-content']}>
         <div className={styles['modal-header']}>
           <span className={styles['modal-title']}>Spiel beendet</span>