fix: kickoff time centered without 'Uhr', unified countdown badge, DevPanel close button

- Kickoff: centered "21:00" above flags (no 'Uhr' suffix)
- Countdown: always rendered as badge (was unstyled span for <60min)
- DevPanel: added close button (✕) in panel header for reliable closing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/DevPanel.tsx

@@ -131,6 +131,7 @@ export default function DevPanel({ currentUser, onUserChange, matches, onRefresh
         <div className={styles.panel}>
           <div className={styles.panelHeader}>
             <span className={styles.panelTitle}>🧪 Simulations-Modus</span>
+            <button className={styles.closeBtn} onClick={() => setOpen(false)}>✕</button>
           </div>
 
           {/* User Switcher */}