Devices: fixes from review round 3

- Attempts carry a generation: one overtaken by a switch, a removal or a login
  change routes nothing back to the old headset and can't report connected.
- Removing the active headset or changing its user/port reroutes at once,
  before anything that can fail.
- One known_hosts file per headset (~/.ssh/frame-control-hosts/<id>):
  forgetting one headset's key can't drop another's, whoever else writes.
- learn() checks, under the config lock, that the block is still what the
  attempt started from before writing to it.
- A switch stops live video and drops captures from the previous headset.
- A probe shares its time between the addresses a name resolves to.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

ui/index.html

@@ -2533,6 +2533,12 @@ function onConnection(s) {
     online = null;
     devGen++;               // answers still on their way are about the other headset
     refreshing = null;      // and the next status check must be a new one
+    if (live) toggleLive(false);  // the other headset's video or captures
+    viewGen++;              // a capture still on its way is the other headset's too
+    lastImg = null; lastShot = null;
+    $("canvas").hidden = true; $("viewerEmpty").hidden = false; $("zoombar").hidden = true;
+    ["stamp", "srcBadge", "asleep"].forEach(id => $(id).hidden = true);
+    $("saveBtn").disabled = true;
     link.reload = true;     // everything on the page was the other headset's
     $("battChip").hidden = true;
   }