- web_server: join the callback-server thread in the start error path so a
failed discovery/URL build doesn't leave a daemon thread running
- web_server: loopback worker now bails if the session was cancelled while
waiting for the callback or exchanging the code, instead of persisting
tokens the user no longer wants (+ regression test)
- onboarding: fall back to window.open when the desktop bridge's
openExternal is unavailable, so the flow never silently stalls
3be9fb7317