system: do not let a failed console request look like an empty answer

fetch() does not throw on 4xx, so a request that came back 401 after the
session had timed out reached the same line as a successful one: the
window showed the prompt and nothing under it, and the input was cleared
as if the command had run. The session lasts 200 seconds, so leaving the
tab open is enough to hit it.

A 401 now goes to the login page instead. The transcript also gets a
trailing newline when the answer lacks one, so the next prompt does not
run into the last line of output, and it is capped so a long session
does not keep growing the node.

html/system.js

@@ -43,7 +43,16 @@ async function cmdSub() {
       method: 'POST',
       body: cmd
     });
-    out.textContent += "> " + cmd + "\n" + await response.text();
+    if (response.status == 401) {
+      window.location.href = 'login.html';
+      return;
+    }
+    let text = await response.text();
+    if (text != "" && !text.endsWith("\n"))
+      text += "\n";
+    if (out.textContent.length > 20000)
+      out.textContent = out.textContent.slice(-16000);
+    out.textContent += "> " + cmd + "\n" + text;
     out.scrollTop = out.scrollHeight;
     input.value = "";
   } catch(err) {