core+gui: chunked multi-QR will transfer (export/import + review/sign wizard, audio channel)

BALQR-framed chunk scheduler (bal/core/qrtransfer.py) with zlib compression,
four chunk presets and a QR_CHUNK_SIZE setting (row 16).
Export/import dialogs (WillQrExportDialog/WillQrImportDialog), per-tx
review-and-sign wizard, export filters (All/Valid/Valid-NC) and an Auto
slideshow with speed + loop for the QR codes; optional audio_modem channel
with a local receive mirror. _prepare_and_sign_tx refactor (no behaviour
change); WillItem.status defaults to '' instead of None (import crash fix);
invalidate_will guard for a missing date_to_check. Docs: PLAN_QR_TRANSFER,
QML_PLAN, README, HANDOFF, CHANGELOG entry 56, AUDIO_MODEM_DEBIAN.

bal/gui/qt/common.py

@@ -28,6 +28,7 @@ from functools import partial
 from typing import Any, Callable, Mapping, Optional, Union
 
 from electrum.bitcoin import NLOCKTIME_BLOCKHEIGHT_MAX, NLOCKTIME_MAX, NLOCKTIME_MIN
+from electrum.gui.common_qt.util import draw_qr
 from electrum.gui.qt.amountedit import BTCAmountEdit
 from electrum.gui.qt.main_window import ElectrumWindow, StatusBarButton
 from electrum.gui.qt.my_treeview import MyTreeView
@@ -42,6 +43,7 @@ from electrum.gui.qt.util import (
     MessageBoxMixin,
     OkButton,
     TaskThread,
+    WaitingDialog,
     WindowModalDialog,
     char_width_in_lineedit,
     getOpenFileName,