* feat(mobile-pairing): surface unpaired-device auth failures instead of silent 4001 loops
Desktop: when a phone repeatedly fails direct-transport E2EE auth with a
token missing from the device registry (pre-v1.4.106 pairing-path bug left
desktops that regenerated their registry rejecting paired phones forever),
throttle to one notification per session and show an actionable toast
pointing at Settings -> Mobile to re-pair.
Mobile: map a bare 4001 close onto the existing auth retry budget (the
encrypted e2ee_error is undecryptable when the desktop keypair changed, so
the close code is the only surviving signal) instead of looping the generic
reconnect forever, and make the auth-failed verdict say 'Pairing invalid -
re-pair with your desktop' instead of a bare 'Auth failed'.
* fix(mobile-pairing): handle stale keys and startup notification races
* fix(mobile-pairing): isolate auth notification failures
* fix(mobile-pairing): keep recovery alert actionable
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