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/ui_test.go

@@ -0,0 +1,27 @@
+package monitor_test
+
+import (
+	"testing"
+	"time"
+
+	"git.eeqj.de/sneak/rtnetmon/internal/monitor"
+)
+
+// TestClockStrings checks that the header shows UTC first and the host's
+// local time second, so the two differ whenever the host is not on UTC.
+func TestClockStrings(t *testing.T) {
+	t.Parallel()
+
+	berlin := time.FixedZone("CET", 3600)
+	now := time.Date(2026, time.November, 6, 20, 30, 0, 0, berlin)
+
+	utc, local := monitor.ClockStrings(now)
+
+	if want := "Fri, 06 Nov 2026 19:30:00 +0000"; utc != want {
+		t.Errorf("utc = %q, want %q", utc, want)
+	}
+
+	if want := "Fri, 06 Nov 2026 20:30:00 +0100"; local != want {
+		t.Errorf("local = %q, want %q", local, want)
+	}
+}