feat: concerts button always visible, shows 'Keine Konzerte angekündigt' when empty

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

styles.css

@@ -858,6 +858,7 @@ button:focus{outline:none}
 .event-book-btn{flex-shrink:0;padding:7px 14px;border-radius:20px;background:var(--accent);color:#fff;font-family:var(--font);font-size:12px;font-weight:700;text-decoration:none;transition:opacity .15s;display:flex;flex-direction:column;align-items:center;gap:2px}
 .event-book-btn:hover{opacity:.85}
 .event-sold-badge{font-size:9px;font-weight:600;opacity:.8;letter-spacing:.3px}
+.events-empty{padding:32px 24px;color:var(--text2);font-size:14px;text-align:center}
 
 /* ── MISC ── */
 .spinner{width:28px;height:28px;border:3px solid var(--surface2);border-top-color:var(--accent);border-radius:50%;animation:spin .8s linear infinite;margin:50px auto;display:block}