isUsable() checked only "is SECRETS_KEY nonempty", so a set-but-malformed
key (wrong length, garbage base64) read back as usable even though
encrypter() rejects it two lines below. EditMailbox::save() and
MailboxTester::run() both gate on isUsable() specifically to avoid an
uncaught RuntimeException reaching the operator; a lying isUsable() meant
that guard did not fire in exactly the configuration it exists for.
Both methods now read resolveKey() — the base64: prefix, the raw-base64
path, the 32-byte check — so they cannot disagree about a value either
one is given. SecretVault::isUsable() and the secrets console page's
"no key" banner both delegate down to this and are covered here too, not
assumed to inherit the fix correctly.
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