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.module.css

@@ -46,6 +46,23 @@
   padding: 12px 16px;
   background: rgba(254,174,50,0.08);
   border-bottom: 1px solid rgba(254,174,50,0.15);
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+}
+
+.closeBtn {
+  background: none;
+  border: none;
+  color: var(--text-muted);
+  font-size: 16px;
+  cursor: pointer;
+  padding: 4px 8px;
+  border-radius: 6px;
+}
+.closeBtn:hover {
+  background: rgba(255,255,255,0.1);
+  color: var(--text-primary);
 }
 
 .panelTitle {