add user session reclaiming

In its current state the implementation should allow a user to resume
their session if the websocket connection is reset, for whatever reason.
This could be expanded to allow session sharing (multiple agents logging
in to the same client), or manual session resume via some sort of
password (encode uid & key to some pass-phrase kinda thing, or QR code).

public/buzzer.css

@@ -1,5 +1,16 @@
 body {
   font-family: sans-serif;
+  overflow: hidden;
+}
+#error {
+  background-color: #fee;
+  border: 3px solid red;
+  padding: 1em;
+  font-family: monospace;
+  display: none;
+}
+#error code {
+  white-space: pre;
 }
 input {
   width: 20em;