tui: use RFC3339-alike format with second precision for "Time"

The current time format (Jan 2, 2006 at 3:04pm (MST)) has a few issues:

 - It displays time at minute granularity, while profiles usually have
   have second-level (and more[^go]) granularity.
 - It uses a 12-hour clock, which is tricky to handle at AM/PM
   switchovers, for people (like myself) who are not used to it. By
   contrast, a 24-hour clock is unambiguous.
 - Minor: using names for months makes it more difficult to calculate
   with time differences.

Hence, I propose replacing it with an RFC3339-alike format, with spaces
between elements for better readability.

[^go]: https://github.com/golang/go/blob/3aa71c12eacd68ec16e7172d92aa5c6af32f0c3b/src/runtime/pprof/proto.go#L351

internal/report/report.go

@@ -1173,7 +1173,7 @@ func ProfileLabels(rpt *Report) []string {
 		label = append(label, "Doc: "+url)
 	}
 	if prof.TimeNanos != 0 {
-		const layout = "Jan 2, 2006 at 3:04pm (MST)"
+		const layout = "2006-01-02 15:04:05 MST"
 		label = append(label, "Time: "+time.Unix(0, prof.TimeNanos).Format(layout))
 	}
 	if prof.DurationNanos != 0 {

internal/report/report_test.go

@@ -21,8 +21,10 @@ import (
 	"path/filepath"
 	"regexp"
 	"runtime"
+	"slices"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/google/pprof/internal/binutils"
 	"github.com/google/pprof/internal/graph"
@@ -595,3 +597,21 @@ func TestDocURLInLabels(t *testing.T) {
 		t.Errorf("expected URL %q not found in %s", url, labels)
 	}
 }
+
+func TestProfileLables(t *testing.T) {
+	// Force the local timezone to UTC for the duration of this function to get a
+	// predictable result out of timezone printing.
+	defer func(prev *time.Location) { time.Local = prev }(time.Local)
+	time.Local = time.UTC
+
+	profile := testProfile.Copy()
+	profile.TimeNanos = time.Unix(131, 0).UnixNano()
+	rpt := New(profile, &Options{
+		SampleValue: func(v []int64) int64 { return v[1] },
+	})
+
+	const want = "Time: 1970-01-01 00:02:11 UTC"
+	if labels := ProfileLabels(rpt); !slices.Contains(labels, want) {
+		t.Errorf("wanted to find a label containing %q, but found none in %v", want, labels)
+	}
+}