feat: show pace (min/km) for running, hiking, walking, other activities

Cycling keeps km/h; pace sports show e.g. "5:30 /km" in the feed card,
activity stat panel (avg/max), and laps table.

site/src/lib/format.ts

@@ -1,5 +1,19 @@
 import type { Privacy, Sport } from './types';
 
+const PACE_SPORTS: Set<Sport> = new Set(['running', 'hiking', 'walking', 'other']);
+
+export function isPaceSport(sport: Sport): boolean {
+  return PACE_SPORTS.has(sport);
+}
+
+export function formatPace(kmh: number | null): string {
+  if (kmh == null || kmh <= 0) return '—';
+  const secsPerKm = 3600 / kmh;
+  const m = Math.floor(secsPerKm / 60);
+  const s = Math.round(secsPerKm % 60);
+  return `${m}:${s.toString().padStart(2, '0')} /km`;
+}
+
 /** True for "unlisted" activities (and the legacy "private" alias).
  *  Use this everywhere instead of comparing against 'private' directly. */
 export function isUnlisted(privacy: Privacy | string | null | undefined): boolean {