Volume move: extract sink-based inner functions

- Extract `move_volumes_with_progress(events, ...)` for cross-volume.
- Extract `move_within_same_volume_with_progress(events, ...)` for same-volume rename.
- Public `move_between_volumes` / `move_within_same_volume` keep their `&AppHandle` signatures, construct `TauriEventSink` in the spawn, and call the inner.
- All in-loop `app.emit(...)` and `emit_progress_via_app(...)` sites in the cross-volume body now go through the sink (`emit_progress_via_sink`, `emit_complete`).
- No behavior change. M1 step 1 + 2 of the transfer-driver refactor (see `docs/specs/transfer-driver-refactor-plan.md`).

Author: vdavid