Add --list-devices flag to show available microphones

[turqoisehex]

Summary

Users with multiple microphones have no way to see which audio input devices hushtype can detect. A --list-devices flag would enumerate all available input devices and exit, helping users choose the right one.

Acceptance Criteria

• Add --list-devices argument to argparse
• When invoked, list all audio input devices with index and name using PyAudio
• Indicate which device would be auto-selected (NVIDIA Broadcast if available, else default)
• Exit after listing (do not start dictation)

Relevant Code

• find_input_device() already iterates devices: hushtype.py line ~107-118
• PyAudio device enumeration: p.get_device_info_by_index(i)

Notes

This is a small, self-contained feature. The device iteration code already exists in find_input_device() -- this issue just adds a CLI flag to expose that information to the user.