Support Bookworm install paths

[hjoungjoo]

Summary

This draft PR is the first split from my Raspberry Pi 4/CM5 Bookworm compatibility branch.

It focuses only on the install/path foundation:

• add shared shell path helpers for the current OS user and data directory
• prefer /boot/firmware/config.txt on Bookworm, with /boot/config.txt fallback
• render systemd/Samba templates with the actual install user and paths
• remove several hard-coded /home/pifinder paths from install/update/migration/runtime helpers
• keep the upstream Rev-4 gpio-poweroff setup while writing to the active boot config path
• make pifinder_setup.sh friendlier to Raspberry Pi OS Bookworm package/service behavior

Out of scope for this PR:

• board-specific GPS/UART defaults
• Bluetooth keyboard UI/input work
• focus/gain display changes
• Korean localization
• MF_PiFinder-specific documentation

Tested

• bash -n pifinder_paths.sh pifinder_setup.sh pifinder_update.sh pifinder_post_update.sh switch-ap.sh switch-cli.sh migration_source/*.sh
• python3 -m compileall -q PiFinder
• python3 -m ruff check PiFinder tests
• python3 -m pytest tests/test_main.py tests/test_menu_struct.py -q
• python3 -m pytest tests/test_sys_utils.py -q

Note

On this Raspberry Pi 4 test system, python3 -m pytest -m smoke currently fails during collection on upstream main because newly added battery/hardware/tetra3 tests import hardware-specific modules before marker selection completes. The targeted smoke files above pass.

[hjoungjoo]

This is part of a larger Raspberry Pi 4/5/CM5 Bookworm compatibility branch that I am splitting into smaller, feature-focused PRs for easier review.

After these initial draft PRs, I plan to split out the GPS/UART board-profile work and the Bluetooth keyboard support into separate PRs.