Fix TUI quit hang on Python 3.13

[balloob]

Summary

• stop using the event loop default executor for blocking readchar.readkey() calls
• move keyboard reads onto a dedicated daemon thread and forward keys back to the asyncio loop
• preserve existing TUI key handling while avoiding hangs during interpreter shutdown

Testing

• ./scripts/run-in-env.sh pre-commit run -a

[Copilot]

Pull request overview

This PR addresses a Python 3.13 shutdown hang in the Sendspin TUI by removing reliance on the asyncio event loop’s default executor for blocking readchar.readkey() calls, instead reading keys on a dedicated background thread and forwarding them into the asyncio loop.

Changes:

• Add a dedicated daemon thread to perform blocking readchar.readkey() reads.
• Forward keypresses back to the asyncio loop via loop.call_soon_threadsafe() and an asyncio.Queue.
• Replace run_in_executor() usage with await key_queue.get() in the main keyboard loop.

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