fix(ui): recover from a dead session instead of stranding the operator

[pyramation]

Summary

#92 made the red dot say "Your session expired or was revoked — sign in again", but there was nothing to sign in with: the header still showed the username, useSocket kept reconnecting with the same dead token, and every retry re-diagnosed the same 401. A dead-end.

Sessions are stored per project, so this is easy to hit now that switching the project in use restarts the brain — the token was minted against the previous project's session list.

A sessionExpired diagnosis (which only fires when /api/config is healthy and /api/me 401s, so the server is definitively rejecting this token) now clears it:

// app.tsx
useEffect(() => {
  if (token && connection.state === 'down' && connection.cause === 'sessionExpired') {
    sessionEnded();  // drop the token → `!user` → <LoginScreen/>
  }
}, [token, connection.state, connection.cause, sessionEnded]);

useAuth grows sessionEnded() (a logout that records why) and remembers the last username in wg_last_user, so the login screen returns with a reason and one field left to fill:

Your session ended — it expired, was revoked, or the show switched project.

rejected deliberately does not sign out: /api/me passed there, so the token is fine and signing out would lose a good session over a server-side problem.

Link to Devin session: https://app.devin.ai/sessions/972698f89f494b86828010666a002b8f
Requested by: @pyramation

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