Move the workout zone range label onto the type scale

The bpm range under each effort zone sat at an arbitrary text-[10px]. It is
a micro-label on a card, not a chart axis, so it takes the text-2xs token
the standards mint for exactly this.

src/components/insights/workout-detail/zones.tsx

@@ -85,7 +85,7 @@ export function WorkoutDetailZones({ workout }: WorkoutDetailZonesProps) {
                 {formatMinutes(z.seconds)}
               </span>
               {z.lowBpm != null ? (
-                <span className="text-muted-foreground text-[10px] tabular-nums">
+                <span className="text-muted-foreground text-2xs tabular-nums">
                   {z.lowBpm}
                   {z.highBpm != null ? `–${z.highBpm}` : "+"}
                 </span>