Add integrated GPS/NTP time sync

[hjoungjoo]

Summary

• add integrated Time Sync monitoring with GPS and NTP sources
• select the best available time source by estimated quality, with GPS/NTP-specific status
• add optional helper-mediated system clock and RTC sync requests
• add software PPS status, Time Sync UI, NTP server selection, and docs

Context

This is split out from the larger MF PiFinder compatibility branch as a focused draft PR. Time Sync is off by default; when enabled it can compare GPS and NTP, and it handles poor NTP/network conditions by reporting unavailable or low-quality NTP rather than blocking normal PiFinder operation.

Testing

• cd python && pytest tests/test_gps_time_sync.py tests/test_gps_time_sync_helper.py tests/test_gps_time_sync_status_ui.py tests/test_gps_time_sources.py -q
• python3 -m py_compile python/PiFinder/gps_time_sync.py python/PiFinder/gps_time_sync_helper.py python/PiFinder/gps_gpsd.py python/PiFinder/gps_ubx.py python/PiFinder/gps_ubx_parser.py python/PiFinder/main.py python/PiFinder/ui/callbacks.py python/PiFinder/ui/gps_time_sync_status.py python/PiFinder/ui/menu_structure.py python/PiFinder/ui/textentry.py
• git diff --check upstream/main...HEAD

Notes

• default remains disabled via time_sync_enabled=false
• NTP custom server entry is handled from the NTP Server > Custom item
• outdoor GPS validation is still planned, so this remains Draft