Block found celebration + smarter ZMQ stale detection

Replace the subtle petal animation with a full block-found party:
confetti rain, sakura petals, ember burst, and a dismissible banner
showing block height and reward. Triggers on pool block finds (not
every network block). Animations loop until the user clicks dismiss.

Fix ZMQ stale banner false-positiving during long block intervals by
comparing ZMQ age against tip-change age instead of a fixed 30-min
threshold. Add tip_changed_age to the snapshot so the UI can tell
"no blocks on the network" from "ZMQ is broken". Use solid background
colors on health banners instead of transparent rgba.

ui/src/lib/HealthBanners.svelte

@@ -13,13 +13,17 @@
     return Math.max(0, (d.block_submit_attempts ?? 0) - (d.block_submits_confirmed ?? 0));
   });
 
-  // ZMQ stale = configured but no event in 30+ minutes. Bitcoin's avg
-  // block interval is 10 min; 30 min covers normal variance without
-  // false-alarming on quiet stretches.
+  // ZMQ stale = the tip has advanced but ZMQ didn't fire. ZMQ
+  // delivers within milliseconds, so if the tip changed recently
+  // but the last ZMQ event is much older, the subscriber is broken.
+  // When no block has been mined, both ages grow together → no alarm.
   const zmqStale = $derived.by(() => {
     const d = snap.data;
     if (!d || !d.zmq_enabled || !d.has_last_zmq_event) return false;
-    return (d.last_zmq_event_age ?? 0) > 1800;
+    const zmqAge = d.last_zmq_event_age ?? 0;
+    const tipAge = d.tip_changed_age ?? 0;
+    // Alarm when ZMQ is 3+ min older than the last tip change.
+    return zmqAge > tipAge + 180;
   });
 </script>
 
@@ -67,8 +71,8 @@
     line-height: 1.4;
   }
   .banner.warn {
-    background: rgba(220, 170, 60, 0.10);
-    border-color: rgba(220, 170, 60, 0.40);
+    background: rgb(50, 40, 15);
+    border-color: rgb(220, 170, 60);
     color: rgb(220, 180, 100);
   }
   .icon {