fix: fallback to direct file fetch in ActivityDetailLoader

If the index-based lookup fails (shard fetch silently failed, stale
index state, etc.), try fetching the activity detail file directly from
each user shard's _merged/activities/ directory. This makes private
activities and newly-synced activities accessible even when the index
resolution fails.

Also add console.error logging when shards fail in resolveShards to
help diagnose root causes.

site/src/components/ActivityDetailLoader.svelte

@@ -2,7 +2,7 @@
   import { onMount } from 'svelte';
   import { loadIndex } from '../lib/dataloader';
   import ActivityDetail from './ActivityDetail.svelte';
-  import type { ActivitySummary } from '../lib/types';
+  import type { ActivitySummary, BASIndex } from '../lib/types';
 
   export let base: string = '/';
 
@@ -10,6 +10,56 @@
   let notFound = false;
   let loading = true;
 
+  /**
+   * Fallback: if the activity isn't found in the loaded index (e.g. the user's
+   * shard fetch failed silently), try to fetch the detail file directly from
+   * each known user shard's _merged/activities/ directory.
+   */
+  async function fetchActivityDirect(id: string): Promise<ActivitySummary | null> {
+    try {
+      const r = await fetch(`${base}data/index.json`);
+      if (!r.ok) return null;
+      const root: BASIndex = await r.json();
+      for (const shard of (root.shards ?? [])) {
+        if (!shard.handle) continue;
+        const url = `${base}data/${shard.handle}/_merged/activities/${id}.json`;
+        try {
+          const dr = await fetch(url);
+          if (!dr.ok) continue;
+          const d = await dr.json();
+          if (d.id !== id) continue;
+          return {
+            id: d.id,
+            title: d.title ?? id,
+            sport: d.sport ?? 'other',
+            sub_sport: d.sub_sport ?? null,
+            started_at: d.started_at ?? '',
+            distance_m: d.distance_m ?? null,
+            duration_s: d.duration_s ?? null,
+            moving_time_s: d.moving_time_s ?? null,
+            elevation_gain_m: d.elevation_gain_m ?? null,
+            avg_speed_kmh: d.avg_speed_kmh ?? null,
+            max_speed_kmh: d.max_speed_kmh ?? null,
+            avg_hr_bpm: d.avg_hr_bpm ?? null,
+            max_hr_bpm: d.max_hr_bpm ?? null,
+            avg_cadence_rpm: d.avg_cadence_rpm ?? null,
+            avg_power_w: d.avg_power_w ?? null,
+            mmp: d.mmp ?? null,
+            source: d.source ?? null,
+            privacy: d.privacy ?? 'public',
+            detail_url: `${shard.handle}/_merged/activities/${id}.json`,
+            track_url: d.bbox && d.privacy !== 'private' && d.privacy !== 'no_gps'
+              ? `${shard.handle}/_merged/activities/${id}.geojson`
+              : null,
+            preview_coords: null,
+            handle: shard.handle,
+          };
+        } catch { /* try next shard */ }
+      }
+    } catch { /* ignore */ }
+    return null;
+  }
+
   onMount(async () => {
     // Extract activity ID from the URL path: /activity/{id}/
     const match = window.location.pathname.match(/\/activity\/([^/]+)/);
@@ -19,6 +69,13 @@
     try {
       const index = await loadIndex(base);
       activity = index.activities.find(a => a.id === id) ?? null;
+
+      if (!activity) {
+        // Shard lookup failed — try fetching the detail file directly.
+        // This handles transient shard fetch errors, stale index state, etc.
+        activity = await fetchActivityDirect(id);
+      }
+
       if (!activity) notFound = true;
     } catch {
       notFound = true;