fix: ternäre Button-Kette in MobileStampScreen korrigiert

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

frontend/src/pages/mobile/MobileStampScreen.tsx

@@ -284,8 +284,6 @@ export function MobileStampScreen() {
               <span className='animate-spin rounded-full h-7 w-7 border-4 border-white border-t-transparent' />
             ) : 'AUSSTEMPELN'}
           </button>
-        )}
-
         ) : null}
 
         {/* Sekundärer Button: Pause starten */}