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/plugin_base.py

@@ -279,6 +279,13 @@ class BalPlugin(BasePlugin):
         # stay display-only outside the wizard unless the user opts in.
         self.EDITABLE_DATES = BalConfig(config, "bal_editable_dates", False)
 
+        # QR_CHUNK_SIZE (will transfer via QR): payload budget, in bytes, used
+        # per QR frame when exporting/importing a will through the QR channel.
+        # The settings dialog offers the 4 standard presets of
+        # bal.core.qrtransfer.CHUNK_PRESETS; this stores the selected budget.
+        # Default 150 (small QR, low-resolution cameras).
+        self.QR_CHUNK_SIZE = BalConfig(config, "bal_qr_chunk_size", 150)
+
         # NUM_REMINDERS (Group D / D1): how many SEPARATE reminder events the
         # exported .ics calendar should contain. Each reminder becomes its own
         # VEVENT (its own date in the calendar). The dates are spread uniformly