Add Bluetooth keyboard support

[hjoungjoo]

Summary

• add a Bluetooth keyboard menu for scan, pair, connect, disconnect, remove, refresh, and reconnect actions
• extend physical keyboard/libinput handling so Bluetooth HID keyboards can send navigation, numeric, long/alt, and text-entry keycodes
• add startup auto-reconnect for paired Bluetooth keyboards without blocking normal PiFinder startup
• add Bluetooth HID input setup and fake sys_utils stubs for local development

Scope

This is split from the larger MF_PiFinder compatibility branch. It focuses on Bluetooth/USB HID keyboard support and intentionally avoids the GPS/UART board-profile, INDI mount, and time-sync work in the other draft PRs.

Validation

• python3 -m py_compile python/PiFinder/keyboard_interface.py python/PiFinder/keyboard_pi.py python/PiFinder/main.py python/PiFinder/sys_utils.py python/PiFinder/sys_utils_fake.py python/PiFinder/ui/base.py python/PiFinder/ui/bluetooth_keyboard.py python/PiFinder/ui/menu_manager.py python/PiFinder/ui/menu_structure.py python/PiFinder/ui/textentry.py python/tests/test_sys_utils.py python/tests/test_keyboard_interface.py
• pytest python/tests/test_keyboard_interface.py python/tests/test_sys_utils.py python/tests/test_menu_struct.py python/tests/test_main.py -q
• git diff --check

More compatibility work is being split into feature-focused draft PRs; GPS/UART board profile support is in #505.