ui: Select environment camera if possible

Apparently, Firefox/Android defaults to using the front camera, and
does not provide any native UI for selecting a different one.  We can
request the back camera by indiciting that the "environment" camera is
"ideal."

js/transaction.ts

@@ -57,7 +57,11 @@ async function startCamera() {
     let stream: MediaStream;
     try {
         stream = await navigator.mediaDevices.getUserMedia({
-            video: true,
+            video: {
+                facingMode: {
+                    ideal: "environment",
+                },
+            },
             audio: false,
         });
     } catch (ex) {