refactor(rtt): use proto Duration, add tests

internal/cli/tui/format.go

@@ -0,0 +1,12 @@
+package tui
+
+import (
+	"fmt"
+	"math"
+	"time"
+)
+
+// FormatRTT formats a round-trip time duration as whole milliseconds, e.g. "140ms".
+func FormatRTT(d time.Duration) string {
+	return fmt.Sprintf("%dms", int64(math.Round(float64(d)/float64(time.Millisecond))))
+}