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/shield_dialog.h

@@ -41,6 +41,11 @@ public:
      * @brief Render the dialog (call each frame)
      */
     static void render(App* app);
+
+    /**
+     * @brief Close the dialog (used by the UI sweep teardown)
+     */
+    static void hide();
 };
 
 } // namespace ui