fix(keys): a device refuses Argon2id costs beyond the register's ceiling

PasswordKdf.fromJson refused costs below the floor but took any above it,
so an instance could ask a device for 100 GiB and crash the app at every
sign-in. It now refuses more than the register's own ceiling (16 passes,
1 GiB; KeyRefusal.excessivePasswordKdf), and the app reads either refusal,
before deriving or sending anything, as PasswordProblem.unsafeCosts with
a text of its own (before, costs below the floor read as keys missing).

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/lib/l10n/halkyon_app_en.arb

@@ -431,5 +431,6 @@
   "passwordUnusable": "This password can't be used: it's empty, or holds characters that aren't text.",
   "passwordInstanceUnproven": "The instance couldn't prove it knows your password, so this device trusts nothing it says. Try again; if it happens again, the instance isn't the one it claims to be.",
   "passwordKeysMissing": "Your account's keys aren't on this device. Sign out and in again with your password here.",
+  "passwordUnsafeCosts": "The instance asked for your password to be checked in a way no account uses, so nothing was sent. If it happens again, the instance isn't the one it claims to be.",
   "passwordFailed": "Couldn't reach the instance. Check the connection and try again."
 }