Use UTC ticks and atomic operations for startup/heartbeat timing to improve watchdog reliability

[CJKmkp]

Motivation

• Replace non-atomic DateTime.Now usage and DateTime globals with UTC tick-based atomic storage to avoid race conditions and timezone/clock issues in startup and watchdog logic.
• Improve reliability of heartbeat/watchdog checks by reading consistent timestamps under multithreaded access.

Description

• Replaced DateTime fields (lastHeartbeat, splashScreenStartTime, appStartupStartTime, startupCompleteHeartbeat) with UTC tick long fields (lastHeartbeatTicksUtc, splashScreenStartTimeTicksUtc, appStartupStartTimeTicksUtc) and use Interlocked.Exchange/Interlocked.Read for thread-safe updates and reads.
• Switched timestamp assignments from DateTime.Now to DateTime.UtcNow and store ticks to the new atomic fields (e.g. when showing splash and at app startup).
• Added helper ReadUtcTicks to convert stored ticks back to DateTime with DateTimeKind.Utc and to interpret default values as DateTime.MinValue.
• Updated the heartbeatTimer and watchdogTimer logic to use atomic tick reads and DateTime.UtcNow to compute elapsed times and decide on silent restarts.

Testing

• No automated tests were run as part of this change.

---

Codex Task