Add Korean UI localization

[hjoungjoo]

Summary

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

It adds Korean as a selectable UI language:

• add Korean gettext catalog files
• add Korean to the language menu and extraction markers
• allow --lang ko for development/testing
• use the existing Sarasa CJK font for Korean, like Chinese
• restart PiFinder after switching to Korean so the CJK font is reloaded

Out of scope for this PR:

• hardware compatibility changes
• install script changes
• Bluetooth keyboard work
• focus/gain changes
• MF_PiFinder-specific documentation

Tested

• python3 -m compileall -q PiFinder
• python3 -m ruff check PiFinder tests
• python3 -c "import gettext; tr=gettext.translation('messages','locale',languages=['ko']); print(tr.gettext('Start')); print(tr.gettext('Korean'))"
• python3 -m pytest tests/test_main.py tests/test_menu_struct.py -q

[jscheidtmann]

Looks good to me.