fix(lite): keep a rejected import key out of the log and Settings

The backend's error for a key it doesn't recognize repeats the key
("'<key>' was not recognized as either a spending key or a viewing
key"). Import passed that text on as its error, so a mistyped private
key was written to the lite log (shown in the Console tab) and to the
Settings status line. A failed import now carries a fixed reason
(unrecognized format, wallet locked, key already exists, could not be
decoded, or rejected), which is all the log records. Settings shows
"Couldn't import the key — check it, unlock the wallet (if encrypted)
and try again." instead, translated in all eight languages.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/util/i18n.cpp

@@ -951,6 +951,7 @@ void I18n::loadBuiltinEnglish()
         strings_["lite_import_key_label"] = "Import key";
         strings_["lite_import"] = "Import";
         strings_["lite_key_imported"] = "Key imported — run a sync to scan its history";
+        strings_["lite_key_import_failed"] = "Couldn't import the key \xE2\x80\x94 check it, unlock the wallet (if encrypted) and try again.";
         strings_["lite_security"] = "Security";
         strings_["lite_encrypt_wallet"] = "Encrypt wallet";
         strings_["lite_wallet_encrypted"] = "Wallet encrypted";