test(sweep): add capture surfaces for the Wave-2 fund/secret modals

Register sweep surfaces for the redesigned Wave-2 modals so they get captured for
visual review: modal-shield + modal-merge (ShieldDialog's two modes),
modal-transfer (a z->t transfer so the converted deshielding DialogWarningHeader
renders), and modal-key-export (KeyExportDialog — named distinctly from the
settings modal-export-key). Each setup opens the dialog through its public show()
without clicking any button, so no async RPC fires; teardown closes it.

Adds a static hide() to ShieldDialog and KeyExportDialog for clean sweep teardown
(AddressTransferDialog already had close()). KeyExportDialog::hide() also clears the
revealed key + frees its QR, mirroring the dialog's own close/dismiss secret-wipe.

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

src/ui/windows/key_export_dialog.cpp

@@ -58,6 +58,16 @@ bool KeyExportDialog::isOpen()
     return s_open;
 }
 
+void KeyExportDialog::hide()
+{
+    s_open = false;
+    s_fetching = false;
+    s_key.clear();
+    s_show_key = false;
+    s_error.clear();
+    releaseQr();
+}
+
 void KeyExportDialog::render(App* app)
 {
     if (!s_open) return;