fix(settings): wipe change-passphrase buffers on dialog dismiss

The Change Passphrase dialog only zeroed change_old/new/confirm_pass_buf_ on
the success path (inside changePassphrase); dismissing via X / Esc / outside-
click left the typed passphrases resident in the input buffers. Add a same-
frame teardown mirroring the encrypt dialog's X-button cleanup: when the
BlurFloat overlay clears show_change_passphrase_ mid-frame, memset the three
buffers. The failure path still keeps them intact for retry, and the async
change already copies the passphrases into worker-owned strings, so wiping the
UI buffers cannot disturb an in-flight change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app_security.cpp

@@ -1401,6 +1401,15 @@ void App::renderEncryptWalletDialog() {
             ImGui::EndDisabled();
             EndOverlayDialog();
         }
+
+        // Wipe the passphrase buffers if the dialog was dismissed (X / Esc /
+        // outside-click) without submitting. The success path already zeroes
+        // them in changePassphrase(); the failure path keeps them for retry.
+        if (!show_change_passphrase_) {
+            memset(change_old_pass_buf_, 0, sizeof(change_old_pass_buf_));
+            memset(change_new_pass_buf_, 0, sizeof(change_new_pass_buf_));
+            memset(change_confirm_buf_, 0, sizeof(change_confirm_buf_));
+        }
     }
 }