feat(playlists): #417 — "Refreshed …" subtitle on system tiles

Closes the last buildable item of the #411 system-playlists-v2
umbrella. System playlists atomic-replace on rebuild, so created_at
(already on the wire — no server change) is the last-rotated time.
Surface it as a small tile subtitle so users see how fresh a mix
is: "Refreshed just now / today / yesterday / N days ago / Mon D".

- web PlaylistCard: refreshedLabel() + a muted footer line, shown
  only when system_variant != null. Unparseable/empty timestamp →
  suppressed (web test fixtures use created_at:'' so no test churn).
- flutter PlaylistCard: mirrored _refreshedLabel() + subtitle under
  the system badge for isSystem playlists.

Friendly wording deliberately distinct from HistoryRow's "m/h ago";
per-surface helper per the project's existing relative-time
convention. CI-pending; closes with the umbrella on device-check.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/lib/components/PlaylistCard.svelte

@@ -17,6 +17,27 @@
   // (#411 R2) — no hardcoded kind list here.
   const refreshable = $derived(playlist.refreshable === true);
 
+  // #417: system playlists atomic-replace on rebuild, so created_at
+  // is the last-rotated time. Surface it so users know how fresh the
+  // mix is. Friendly wording, not the HistoryRow "m/h ago" style.
+  function refreshedLabel(iso: string): string {
+    const t = new Date(iso);
+    if (Number.isNaN(t.getTime())) return '';
+    const now = new Date();
+    const mins = Math.floor((now.getTime() - t.getTime()) / 60000);
+    if (mins < 5) return 'Refreshed just now';
+    const startOfToday = new Date(now);
+    startOfToday.setHours(0, 0, 0, 0);
+    const days = Math.floor((startOfToday.getTime() - t.getTime()) / 86400000) + 1;
+    if (t.getTime() >= startOfToday.getTime()) return 'Refreshed today';
+    if (days <= 1) return 'Refreshed yesterday';
+    if (days < 7) return `Refreshed ${days} days ago`;
+    return `Refreshed ${t.toLocaleDateString(undefined, { month: 'short', day: 'numeric' })}`;
+  }
+  const refreshed = $derived(
+    playlist.system_variant != null ? refreshedLabel(playlist.created_at) : ''
+  );
+
   const queryClient = useQueryClient();
 
   let starting = $state(false);
@@ -153,6 +174,9 @@
           · by {playlist.owner_username}
         {/if}
       </div>
+      {#if refreshed}
+        <div class="truncate text-xs text-text-muted">{refreshed}</div>
+      {/if}
     </div>
   </a>
 </div>