system: show what the console command printed

The console tab posts to /cmd and throws the response away, so the page stayed
silent whether the command worked, was refused or was just misspelt. Now that
/cmd answers with the command's output, the page keeps a running transcript:
each command echoed after a prompt, then whatever it printed.

The box is a pre element filled through textContent, so nothing the device
prints is taken for markup. It scrolls to the newest line and the input clears
after a send.

html/system.js

@@ -35,14 +35,19 @@ async function ipSub() {
 }
 
 async function cmdSub() {
-  var cmd = document.getElementById('console_cmd').value;
+  const input = document.getElementById('console_cmd');
+  const out = document.getElementById('console_out');
+  const cmd = input.value;
   try {
     const response = await fetch('/cmd', {
       method: 'POST',
       body: cmd
     });
-    console.log('Completed!', response);
+    out.textContent += "> " + cmd + "\n" + await response.text();
+    out.scrollTop = out.scrollHeight;
+    input.value = "";
   } catch(err) {
+      out.textContent += "> " + cmd + "\n" + err + "\n";
       console.error(`Error: ${err}`);
   }
 }