Finish Fn dictation with another key

[quanru]

Summary

• Finish active dictation when another key is pressed while Fn is the trigger.
• Keep Escape cancellation and existing Fn tap and hold gestures.
• Document that the finishing key still reaches the foreground app.

Verification

• PYTHONPATH=src:. .venv/bin/python -m unittest discover -s tests/unit -t . (348 passed)
• git diff origin/main...HEAD --check

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