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An aged-out session (14 days since loggedInAt) reads as logged-out, so
getSigningMode() falls back to `local` and the env-var check blames a
missing ACURAST_MNEMONIC — true, but it points at the wrong fix for
someone who was logged in and just needs `acurast login` again.

- authStore: split parse() out of read(), add getExpiredAuth() to surface
  a stored-but-expired record (project pin first, matching getActiveAuth)
- signingNotice: expiredSessionMessage() names the account, its age, and
  both ways out; omits the age when loggedInAt is unparseable
- deploy: check for an expired session before validateDeployEnvVars, so
  both `deploy` and `deploy vps` (shared executeDeployFlow) report it

The genuinely-logged-out case still gets the ACURAST_MNEMONIC error.