Show UTC as the first header clock instead of hardcoded Los Angeles time

The header drew the host's local time and then America/Los_Angeles
time, so on a Mac in that zone both clocks read the same and neither
was UTC. The first clock is now UTC and the second the host's local
time, which also drops the hardcoded zone and its load-failure path.

internal/monitor/export_test.go

@@ -2,6 +2,7 @@ package monitor
 
 import (
 	"context"
+	"time"
 
 	"git.eeqj.de/sneak/rtnetmon/internal/starlink"
 )
@@ -59,6 +60,9 @@ func (m *Monitor) TCPHosts() []string { return m.tcpHosts }
 // Interfaces returns the monitored interfaces.
 func (m *Monitor) Interfaces() []*InterfaceStatus { return m.interfaces }
 
+// ClockStrings exposes clockStrings for external tests.
+func ClockStrings(now time.Time) (string, string) { return clockStrings(now) }
+
 // PingArgs exposes pingArgs for external tests.
 func PingArgs(goos, iface, host string) []string { return pingArgs(goos, iface, host) }