Devices: fixes from review round 2

- Switching headsets is serialized with the start of any install; background
  work counts as running from before its thread starts. use() reroutes every
  command at once and makes ensure() wait for the new headset.
- A new user or port reroutes commands even if the attempt then fails.
- ~/.ssh/config edits take a lock file shared with Set Up Connection
  (frame_connect.py and connect.sh, which now also writes atomically).
- A finished attempt no longer writes its older settings over a change Set
  Up Connection made meanwhile.
- Pin edits are locked and swapped atomically.
- A bare alias behind ProxyJump/ProxyCommand is left to ssh to reach.
- The page drops answers about the previous headset after a switch; the
  header switcher takes clicks in the macOS title bar.

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

app/main.js

@@ -189,7 +189,7 @@ async function restartServer() {
 // On macOS the page's sticky header becomes the title bar, clear of the traffic lights.
 const CHROME_CSS = IS_MAC && `
   header { padding-left: 92px !important; -webkit-app-region: drag; user-select: none; }
-  header a, header button, header input, header .chip { -webkit-app-region: no-drag; }
+  header a, header button, header input, header select, header .chip { -webkit-app-region: no-drag; }
 `;
 
 // Restart Server can start a new load while an older one is still waiting for