M5Unified: Sync RTC using UTC time

[m5agent]

Fixes #117.

M5.begin() reads the RTC as UTC through M5Unified::setSystemTimeFromRtc(), but rtc_sync() wrote localtime_r() back into the RTC. With a non-UTC timezone every boot added the UTC offset to the RTC and then to the system clock.

Use gmtime_r() so the RTC stays UTC consistently with the read path and machine.RTC().datetime().

Verification:

• StopWatch with RX8130 and a UART-REPL build, no PoE and no StamPLC-specific code:
  • baseline: set the RTC to 11:15:00 UTC; after M5.begin() the RTC registers changed to 20:15:00 local time (UTC+9).
  • fixed: set the RTC to 04:11:15 UTC; two consecutive M5.begin() calls left both the system time and RTC registers at 04:11:15 UTC.
• Full M5STACK_StamPLC firmware build passes against 69846518 with ESP-IDF v5.5.4.
• The original StopWatch Wi-Fi image was restored and verified byte-for-byte at the application partition after the test.

[m5agent]

Detailed verification record:
https://github.com/m5agent/agent-work-logs/blob/6367ac3cf17d3978ae18ca3aebf0c633b8f76b32/records/m5stack-github/2026-10-10-uiflow-issue-117-rtc-utc/README.md