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/lists.py

@@ -667,7 +667,9 @@ class PreviewList(MyTreeView, MessageBoxMixin):
         export_menu.addAction(_("All"), self.export_will)
         export_menu.addAction(_("Valid"), self.export_will_valid)
         export_menu.addAction(_("Valid NC"), self.export_will_valid_incomplete)
+        export_menu.addAction(_("QR Codes"), self.export_will_via_qr)
         menu.addAction(_("Import"), self.import_will_into_details)
+        menu.addAction(_("Import via QR"), self.import_will_via_qr)
         menu.addAction(_("Merge"), self.merge_will)
         menu.addAction(_("Broadcast"), self.broadcast)
         menu.addAction(_("Check"), self.check)
@@ -769,6 +771,12 @@ class PreviewList(MyTreeView, MessageBoxMixin):
     def import_will_into_details(self):
         self.bal_window.import_will_into_details()
 
+    def export_will_via_qr(self):
+        self.bal_window.export_will_via_qr()
+
+    def import_will_via_qr(self):
+        self.bal_window.import_will_via_qr()
+
     def merge_will(self):
         self.bal_window.merge_will_ui()