To accomodate all three points:
The file dialog is a setting in the browser.
For the button/save part assign the following function, boiled down from other answers, to your buttons onclick:
function DownloadCanvasAsImage(){
let downloadLink = document.createElement('a');
downloadLink.setAttribute('download', 'CanvasAsImage.png');
let canvas = document.getElementById('myCanvas');
let dataURL = canvas.toDataURL('image/png');
let url = dataURL.replace(/^data:image\/png/,'data:application/octet-stream');
downloadLink.setAttribute('href', url);
downloadLink.click();
}
Another, somewhat cleaner, approach is using Canvas.toBlob():
function DownloadCanvasAsImage(){
let downloadLink = document.createElement('a');
downloadLink.setAttribute('download', 'CanvasAsImage.png');
let canvas = document.getElementById('myCanvas');
canvas.toBlob(function(blob) {
let url = URL.createObjectURL(blob);
downloadLink.setAttribute('href', url);
downloadLink.click();
});
}
Neither solution is 100% cross browser compatible, so check the client