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/core/will.py

@@ -1338,7 +1338,7 @@ class WillItem(Logger):
             self.tx = Will.get_tx_from_any(w["tx"])
             self.heirs = w.get("heirs", None)
             self.we = w.get("willexecutor", None)
-            self.status = w.get("status", None)
+            self.status = w.get("status") or ""
             self.description = w.get("description", None)
             self.time = w.get("time", None)
             self.change = w.get("change", None)