Fix pace/elapsed-time formatting rounding 59.6s up to ":60" instead of carrying over

formatPaceShort and formatElapsed each floored the minutes and rounded the
leftover seconds independently, so e.g. 419.6 sec/km displayed as "6:60/km"
instead of "7:00/km". Both now round the total seconds once, then split
into minutes/seconds from that rounded value.

frontend/src/components/charts/ExpectedVsActualChart.tsx

@@ -14,20 +14,27 @@ function paceSecPerKm(mps: number | null): number | null {
   return pace <= MAX_PLAUSIBLE_PACE_SEC_PER_KM ? pace : null;
 }
 
-function formatPaceShort(secPerKm: number): string {
-  const m = Math.floor(secPerKm / 60);
-  const s = Math.round(secPerKm % 60);
+// Rounds the total seconds first, then splits into minutes/seconds -- doing
+// it the other way around (floor the minutes, round the leftover seconds
+// separately) can round 59.6s up to "60", displaying e.g. "6:60" instead of
+// carrying over to "7:00".
+function formatMinutesSeconds(totalSeconds: number): string {
+  const total = Math.round(totalSeconds);
+  const m = Math.floor(total / 60);
+  const s = total % 60;
   return `${m}:${s.toString().padStart(2, "0")}`;
 }
 
+function formatPaceShort(secPerKm: number): string {
+  return formatMinutesSeconds(secPerKm);
+}
+
 function formatPace(secPerKm: number): string {
   return `${formatPaceShort(secPerKm)}/km`;
 }
 
 function formatElapsed(minutes: number): string {
-  const m = Math.floor(minutes);
-  const s = Math.round((minutes - m) * 60);
-  return `${m}:${s.toString().padStart(2, "0")}`;
+  return formatMinutesSeconds(minutes * 60);
 }
 
 function percentile(sortedAsc: number[], p: number): number {