fix: wire concurrent transfer setting into worker pool

[Orinks]

Summary

• replace queue sentinels with a resizable worker pool that reuses the shared transfer queue
• protect transfer job and job-list mutations with the service lock so concurrent workers do not race UI observers
• add coverage for concurrent workers and live worker-pool replacement while transfers are active

Testing

• PYTHONPATH=src python3 -m pytest tests/test_transfer_service.py tests/test_app.py -q
• PYTHONPATH=src python3 -m pytest tests/test_transfer_service.py tests/test_queue_during_transfer.py tests/test_transfer_dialog.py tests/test_transfer_persist.py tests/test_retry_failed_transfers.py tests/test_resume_download.py tests/test_transfer_symlink.py tests/test_app.py -q
• ruff check src tests

[Orinks]

Closing — duplicate of #110 which already merged and fixed issue #107. The concurrent transfer worker pool fix is live in dev.