Improve focus preview and camera gain control

[hjoungjoo]

Summary

This draft PR is a focused split from my Raspberry Pi 4/CM5 compatibility branch.

It contains only camera focus/gain behavior:

• add a runtime Camera Gain menu next to Camera Exp
• allow set_gain:profile to return to the camera profile default gain
• show the current runtime gain in the gain menu selection state
• avoid applying dark-sky stretch to bright focus frames
• add a raw-frame display fallback for bright/flat focus preview frames

Out of scope for this PR:

• Bookworm install/path changes
• board-specific GPS/UART defaults
• Bluetooth keyboard work
• Korean localization
• MF_PiFinder-specific documentation

Tested

• python3 -m compileall -q PiFinder
• python3 -m ruff check PiFinder tests
• python3 -m pytest tests/test_main.py tests/test_menu_struct.py -q