fix(security): warn that the seed-backup file is unencrypted plaintext (W4-5)

The seed-phrase "Save" already wrote the file 0600 and zeroed the in-memory buffer, but
the success message was a bare "Saved to <path>" — no hint that it's a permanent
UNENCRYPTED copy of the seed at a predictable location. The message now reads
"Saved an UNENCRYPTED seed file — move it to secure offline storage and delete this
copy: <path>". English source updated; the res/lang back-fill of this changed key is
deferred to the batch i18n pass.

Also documents W5-3 (lite create-time passphrase) as a product decision rather than a
speculative change: the field is already wiped on every path (minimal security risk),
but the labeled masked "passphrase" input at lite create/open/restore is never consumed
by the backend — so either remove the dead UI or wire it into the lite encrypt flow.

Finishes the actionable part of the wallet-hardening P0-A cluster (docs/wallet-hardening.md).
Build-clean; ctest 1/1.

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

src/util/i18n.cpp

@@ -320,7 +320,7 @@ void I18n::loadBuiltinEnglish()
         strings_["seed_backup_load_failed"] = "Could not load the seed phrase.";
         strings_["seed_backup_copy"] = "Copy";
         strings_["seed_backup_save"] = "Save to file…";
-        strings_["seed_backup_saved"] = "Saved to ";
+        strings_["seed_backup_saved"] = "Saved an UNENCRYPTED seed file — move it to secure offline storage and delete this copy: ";
         strings_["seed_backup_save_failed"] = "Could not write ";
         strings_["seed_backup_close"] = "Close";
         strings_["seed_backup_reminder"] = "Your wallet has a 24-word recovery seed phrase. Back it up now in Settings → Node & Security.";