perf: skip feed index fetch when navigating from activity feed

Write the activity summary to sessionStorage on click in ActivityFeed,
then read it synchronously at module init in ActivityDetailLoader so the
page renders immediately without the "Loading activity…" blank screen
or the 2 round-trip index fetch.

Direct URL / bookmark / shared link falls through to the existing slow
path unchanged.

site/src/components/ActivityFeed.svelte

@@ -350,6 +350,7 @@
               <a
                 href={a.detail_url ? `${import.meta.env.BASE_URL}activity/${a.id}/` : `${import.meta.env.BASE_URL}activity/local/?id=${a.id}`}
                 class="before:absolute before:inset-0 before:content-[''] truncate"
+                on:click={() => { try { sessionStorage.setItem(`bincio:activity:${a.id}`, JSON.stringify(a)); } catch {} }}
               >{a.title}</a>
             </h3>
           </div>