feat(activity): show visible creation date on each row

Add a "Lagt til DD.MM.YYYY" line to ActivityRow. For public rows the
existing owner-attribution snippet folds into the same muted footer
line so it reads naturally ("Lagt til 25.05.2026 av Ole-Morten").
Private rows just get the date — they have no public attribution.

Date-only is intentional (per spec: "Det holder med dato"). created_at
is epoch milliseconds, not seconds like scheduled_at, so a separate
formatDateOnly helper avoids confusing the two.

frontend/src/components/ActivityRow.svelte

@@ -51,6 +51,15 @@
     });
   }
 
+  // created_at is epoch *milliseconds* (Date.now() on the server). The
+  // existing formatDate above takes seconds because scheduled_at is stored
+  // that way. Date-only is enough for "added on" — time of day is noise.
+  function formatDateOnly(epochMs: number): string {
+    return new Date(epochMs).toLocaleDateString('nb-NO', {
+      year: 'numeric', month: '2-digit', day: '2-digit',
+    });
+  }
+
   /**
    * OpenStreetMap link for a location.
    *  - If coordinates are present → /?mlat=…&mlon=…&zoom=15 (shows a pin)
@@ -229,6 +238,7 @@
         {@render locationLine(decrypted.loc_label, null, null)}
       {/if}
       {#if decrypted.scheduled_at}<p class="muted">🕒 {formatDate(decrypted.scheduled_at)}</p>{/if}
+      <p class="muted" style="font-size: 0.8rem;">Lagt til {formatDateOnly(activity.created_at)}</p>
     {:else if !session.dek}
       <h3 id={`act-${activity.id}-h`} style="display: flex; align-items: center;">
         <span class="vis-badge private">Privat</span>
@@ -276,17 +286,18 @@
       {@render locationLine(activity.loc_label, activity.loc_lat, activity.loc_lng)}
     {/if}
     {#if activity.scheduled_at}<p class="muted">🕒 {formatDate(activity.scheduled_at)}</p>{/if}
-    {#if activity.visibility === 'public' && activity.owner_display}
-      <p class="muted" style="font-size: 0.8rem;">
-        Lagt til av
+    <p class="muted" style="font-size: 0.8rem;">
+      Lagt til {formatDateOnly(activity.created_at)}
+      {#if activity.visibility === 'public' && activity.owner_display}
+        av
         {#if activity.owner_username}
           <a href={`/${activity.owner_username}/liste`}
              onclick={(e) => onSpaLink(e, `/${activity.owner_username}/liste`)}>{activity.owner_display}</a>
         {:else}
           {activity.owner_display}
         {/if}
-      </p>
-    {/if}
+      {/if}
+    </p>
   {/if}
 
   <div class="row" style="margin-top: 0.5rem;">