fix(security): color seed-save by outcome; explain the Encrypt button; friendly key-export errors

- Seed-backup dialog: a failed "Save to file" was drawn in the success (green)
  colour; it's now red on failure, green on success (flag reset on close).
- Encrypt-wallet dialog: the disabled Encrypt button now says why once the user
  starts typing ("Use at least 8 characters." / "The passphrases don't match."),
  and a "Show passphrase" checkbox reveals both fields.
- Key-export dialog: the raw RPC exception (e.g. "...walletpassphrase first")
  is routed through util::friendlyRpcError(), so a locked wallet reads
  "Unlock your wallet first, then try again." instead of daemon jargon.

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

src/util/i18n.cpp

@@ -697,6 +697,9 @@ void I18n::loadBuiltinEnglish()
         // --- Encrypt / change-passphrase dialogs (settings) ---
         strings_["enc_desc"] = "Encrypting your wallet protects your private keys with a passphrase. After encryption, the daemon will restart.";
         strings_["enc_confirm"] = "Confirm:";
+        strings_["enc_show_pass"] = "Show passphrase";
+        strings_["enc_min_len"] = "Use at least 8 characters.";
+        strings_["enc_mismatch"] = "The passphrases don't match.";
         strings_["enc_encrypting"] = "Encrypting wallet...";
         strings_["enc_wait"] = "Please wait, do not close the application.";
         strings_["enc_success"] = "Wallet encrypted successfully!";