fix(app): an Apple device's keychain gives the keys only while it is unlocked

The reading of the full reading's fixes found that first_unlock_this_device
left the account's private keys readable on a locked phone once it had
been unlocked after starting, which is what forensic tools extract from a
stolen one; and that, the accessibility being part of every keychain
lookup, an entry kept with another class stayed unseen and made the next
write fail as a duplicate, so that person couldn't finish signing in on
that device.

As the developer decided, the keys are kept unlocked_this_device (still
never in a backup nor in iCloud Keychain); nothing decrypts with them
while the device is locked. Keeping keys deletes the entry first, which a
delete finds whatever its class.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_app/README.md

@@ -65,8 +65,9 @@ Written as the apps are: Dart 3.13's primary constructors, Material from
   sodium's are views of memory freed with the isolate that made them) and an
   `AccountKeyStore` (`SecureAccountKeyStore`: the account's unlocked keys in
   the system's keyring, per instance and person, gone once another person
-  signs in or nobody; on iOS and macOS `first_unlock_this_device`, so never
-  in a backup restored elsewhere nor in iCloud Keychain). Signing in (SRP's M2 checked and the keys opened before
+  signs in or nobody; on iOS and macOS `unlocked_this_device`, so readable
+  only while the device is unlocked, never in a backup restored elsewhere nor
+  in iCloud Keychain, each write deleting the entry before it). Signing in (SRP's M2 checked and the keys opened before
   anything is trusted), signing up (the recovery phrase shown once and three of
   its words typed back before anything reaches the instance), a setup never
   finished, the authenticators (QR codes drawn with pub.dev's `qr`), recovery