unify single user and multi user behaviour

site/src/components/AthleteView.svelte

@@ -7,6 +7,10 @@
   import { loadIndex, loadAthlete } from '../lib/dataloader';
 
   export let base: string = '/';
+  /** Explicit index URL for multi-user per-user pages (user's shard). */
+  export let indexUrl: string = '';
+  /** Explicit athlete.json URL for multi-user per-user pages. */
+  export let athleteUrl: string = '';
 
   let athlete: AthleteJson | null = null;
   let activities: ActivitySummary[] = [];
@@ -34,8 +38,8 @@
     mounted = true;
     try {
       const [athleteData, index] = await Promise.all([
-        loadAthlete(import.meta.env.BASE_URL),
-        loadIndex(import.meta.env.BASE_URL),
+        loadAthlete(import.meta.env.BASE_URL, athleteUrl || undefined),
+        loadIndex(import.meta.env.BASE_URL, indexUrl || undefined),
       ]);
       if (!athleteData) throw new Error('athlete.json not found — run bincio extract first');
       athlete = athleteData;