refactor: deduplicate modal/button styles and enforce global utility usage

- Consolidated all modal-related styles into Modal.module.css; ValidationModal.module.css is now deprecated
- All main action/navigation buttons in NewGame and GameDetail use global .btn/.nav-buttons utility classes
- Removed duplicate utility classes from component CSS files
- Fixed .fullscreenToggle class naming for consistency
- Cleaned up component CSS to only contain component-specific styles
- Updated GameCompletionModal to use shared modal styles

This ensures DRY, maintainable, and consistent styling across the app.

src/components/GameCompletionModal.jsx

@@ -1,4 +1,5 @@
 import { h } from 'preact';
+import modalStyles from './Modal.module.css';
 import styles from './GameCompletionModal.module.css';
 
 /**
@@ -21,13 +22,13 @@ const GameCompletionModal = ({ open, game, onConfirm, onClose }) => {
     ? `Unentschieden zwischen ${winners.join(' und ')}`
     : `${winners[0]} hat gewonnen!`;
   return (
-    <div id="game-completion-modal" className="modal show" role="dialog" aria-modal="true" aria-labelledby="completion-modal-title">
-      <div className={styles['modal-content']}>
-        <div className={styles['modal-header']}>
-          <span className={styles['modal-title']} id="completion-modal-title">Spiel beendet</span>
-          <button className={styles['close-button']} onClick={onClose} aria-label="Schließen">×</button>
+    <div id="game-completion-modal" className={modalStyles['modal'] + ' ' + modalStyles['show']} role="dialog" aria-modal="true" aria-labelledby="completion-modal-title">
+      <div className={modalStyles['modal-content']}>
+        <div className={modalStyles['modal-header']}>
+          <span className={modalStyles['modal-title']} id="completion-modal-title">Spiel beendet</span>
+          <button className={modalStyles['close-button']} onClick={onClose} aria-label="Schließen">×</button>
         </div>
-        <div className={styles['modal-body']}>
+        <div className={modalStyles['modal-body']}>
           <div className={styles['final-scores']}>
             {playerNames.map((name, idx) => (
               <div className={styles['final-score']} key={name + idx}>
@@ -38,7 +39,7 @@ const GameCompletionModal = ({ open, game, onConfirm, onClose }) => {
           </div>
           <div className={styles['winner-announcement']}><h3>{winnerText}</h3></div>
         </div>
-        <div className={styles['modal-footer']}>
+        <div className={modalStyles['modal-footer']}>
           <button className={styles['btn'] + ' ' + styles['btn--warning']} onClick={onConfirm} aria-label="Bestätigen">Bestätigen</button>
           <button className={styles['btn']} onClick={onClose} aria-label="Abbrechen">Abbrechen</button>
         </div>