mobile app2 payment history2 add doctor1 ai invoice1 android flutter app1 appointment history1 billing management1 billing system1 book appointment1 customer management1 dark mode1 flutter1 flutter 31 flutter app1 flutter template1 flutter ui kit1 flutter31 invoice app1 invoice generator1 invoice preview1 invoice tracking1 ios flutter app1 make payment1 modern ui1 pdf export1 responsive design1 shopping app1 ui kit1