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.

HANDOFF.md

@@ -392,3 +392,30 @@ See Section 5 for details.
    push to `origin/main`, then run `./make-release.sh` to create the Gitea
    **Release** with the ZIP + signatures attached (it becomes the owner's
    "Latest" download). Always give the owner the Release URL.
+
+### In progress: QR / audio will transfer (branch `feature/bal-qr-transfer`)
+
+- P0–P4 of `PLAN_QR_TRANSFER.md` are implemented on
+  `feature/bal-qr-transfer` and un-committed: core scheduler
+  (`bal/core/qrtransfer.py`), settings row, export/import dialogs +
+  review/sign wizard + lists/window wiring, all with tests
+  (`tests/test_core_qr_transfer.py`, `tests/test_gui_qr_transfer.py`) and
+  docs (README, CHANGELOG entry 56, QML_PLAN, this file).
+- Remaining: **P6** release hygiene — `python3 build_zip.py`, external-zip
+  test, full ruff check (no NEW violations; baseline is intentionally dirty),
+  and a manual on-device walkthrough of the QR path. The audio path is
+  optional and hidden until the `audio_modem` plugin + `amodem` are
+  installed.
+- Commit policy unchanged: nothing is committed until the owner explicitly
+  confirms.
+
+**Dev-box audio prerequisites (audio_modem channel):**
+See `AUDIO_MODEM_DEBIAN.md` — the full Debian setup + verification, with the
+two pitfalls (unversioned `libportaudio.so`, numpy>=2 `tostring` removal):
+- `sudo ln -s /usr/lib/x86_64-linux-gnu/libportaudio.so.2 /usr/lib/x86_64-linux-gnu/libportaudio.so`
+  (the unversioned name the plugin loads; Debian ships only `.so.2`).
+- numpy>=2 patch in `electrum/env/.../amodem/common.py`: `tostring()` →
+  `tobytes()` (already applied locally). Both are runtime-env fixes, not repo
+  changes; see CHANGELOG entry 56.
+- To loop-test on one machine without speakers/mic: during receive,
+  `pactl set-default-source <sink>.monitor` (restore after).