Address review feedback on the download-progress change: decompress and read
sets files off the UI thread, cancel the load/import workers before the
wizard can tear down the importer, and show an 'Extracting file...' status
plus a clean 100% tail so the poll never looks stuck. Quitting Cockatrice
while a card database update runs now asks for confirmation.
Card database updates ran invisibly: MTGJSON parsing spun an indeterminate
bar on the UI thread and the set import blocked the window, while the
onboarding wizard spawned `oracle -b` with no progress to show at all.
- Add byte-level scan progress to `RawJson::scanSetRanges` via an optional
callback, throttled to ~100 reports per scan.
- Emit `OracleImporter::dataReadProgress` during the scan and import sets on a
worker thread, driving the wizard's progress bar per set.
- With `-b`, write machine-readable `PROGRESS <stage> <done> <total>` lines to
stdout for the download/scan/import stages; stderr keeps the log output.
- Parse the oracle stdout in `MainWindow` and forward it to the onboarding
wizard, giving the card database step a determinate bar with stage-specific
status text.
- Guard the async workers against the wizard being closed mid-run.
- Add Google Test coverage for scan progress reporting.
