feat(transfer): wire concurrent transfers setting into worker pool

[Orinks]

Summary

Closes #95

• TransferService (src/portkeydrop/services/transfer_service.py): new service owning the transfer queue and a single daemon worker thread. Defines TransferJob dataclass with status lifecycle (pending → in_progress → complete/failed/cancelled).
• TransferDialog refactored into a stateless observer — closing/hiding it never cancels a running transfer. Added "Send to Background" button (hides dialog) and "Cancel Transfer" button (explicit cancel).
• MainFrame announces completion and failure via screen reader (_announce) even when the dialog is hidden, satisfying the Prism accessibility requirement.
• Comprehensive test suite: 37 new tests covering TransferService queue/worker, cancel semantics, recursive transfers, progress calculation, MainFrame announce on complete/fail, and dialog close behavior.

Acceptance Criteria

• Closing the transfer dialog does not interrupt the transfer
• "Send to Background" button hides dialog; transfer continues
• Transfer completes/fails in background and notifies MainFrame
• Prism announces completion and failure even when dialog is hidden
• TransferService owns the queue and worker thread
• TransferDialog is a stateless observer that can be opened/closed freely

Test plan

• ruff check and ruff format pass
• 589 tests pass (552 existing + 37 new)
• Manual: start a transfer, close dialog, verify transfer completes
• Manual: "Send to Background" hides dialog, transfer continues
• Manual: screen reader announces completion/failure when dialog hidden

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