util: Add GetTimeMillis convenience wrapper for chrono timestamps

Add a convenience wrapper function for obtaining the current system time
in milliseconds. This uses the modern std::chrono library internally
while providing a shorter, more readable interface for code that needs
millisecond precision timestamps.

This improves code readability in areas like the TrafficGraphWidget that
work with millisecond timestamps while maintaining compatibility with
Bitcoin Core's move toward using std::chrono-based time handling.

Author: rebroad

src/util/time.cpp

@@ -75,6 +75,13 @@ void MockableSteadyClock::ClearMockTime()
 
 int64_t GetTime() { return GetTime<std::chrono::seconds>().count(); }
 
+int64_t GetTimeMillis()
+{
+    return std::chrono::duration_cast<std::chrono::milliseconds>(
+        std::chrono::system_clock::now().time_since_epoch()
+    ).count();
+}
+
 std::string FormatISO8601DateTime(int64_t nTime)
 {
     const std::chrono::sys_seconds secs{std::chrono::seconds{nTime}};

src/util/time.h

@@ -96,6 +96,12 @@ using MillisecondsDouble = std::chrono::duration<double, std::chrono::millisecon
  */
 int64_t GetTime();
 
+/**
+ * Convenience wrapper for getting current time in milliseconds.
+ * Internally uses std::chrono::system_clock for consistency with other time functions.
+ */
+int64_t GetTimeMillis();
+
 /**
  * DEPRECATED
  * Use SetMockTime with chrono type