fix(node,client): survive a transient hub state on login, and surface a failed node-key linkHEADmain

Two defensive gaps turned a routine reset-and-reonboard into "impossible de
démarrer le node":

1. daemon._login_with_retry retried a 401 (node key not linked yet) but `raise`d
   on every other status, so a 429 — the daemon's own 5s retries hitting the
   sign-in rate limit — or a 502/503 while the hub restarts during a deploy
   killed the process, and systemd crash-looped it. Those statuses (429, 5xx)
   are now retried with a back-off that respects Retry-After, so a freshly
   reset node stays alive (the operator needs it up to read its key) instead of
   dying. A genuine 4xx (400/422) still raises.

2. create-group's linkNodeKey swallowed every error as "already linked or same
   key" — but PUT /me/node_key is idempotent and returns 200 on a re-link, so
   there was no benign error to hide: the catch only ever hid a real failure
   (a rejected session, a bad key), letting the wizard proceed against a node
   that looked linked but was not, which then could not authenticate. The link
   failure now surfaces (detectNode shows it).

test_login_retry_is_resilient.py holds the retry behaviour (429/5xx retried,
Retry-After honoured, 401 stays alive, 400 still raises); red before, green
after. common/node/hub suites green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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