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.

api/internal/state/aggregator.go

@@ -87,6 +87,7 @@ type Snapshot struct {
 	ZMQEnabled        bool    `json:"zmq_enabled"`
 	LastZMQEventAge   float64 `json:"last_zmq_event_age,omitempty"` // seconds; >=0
 	HasLastZMQEvent   bool    `json:"has_last_zmq_event"`
+	TipChangedAge     float64 `json:"tip_changed_age"`              // seconds since tip height last changed
 
 	// Share counters: raw counts (1 submission = 1 share regardless of diff).
 	// Session = since ckpool started; AllTime = persisted across restarts.
@@ -214,6 +215,12 @@ type Aggregator struct {
 	zmqEnabled       bool
 	lastZMQEventTime time.Time
 
+	// Tip-change tracking: the height and time when we last saw the
+	// chain tip advance. Used to distinguish "ZMQ stale" from "no
+	// blocks on the network".
+	lastTipHeight    int64
+	lastTipChangedAt time.Time
+
 	// All-time share counters (raw, not diff-weighted). Accumulated
 	// using the same delta-integration pattern as cumulative_shares.
 	allTimeAccepted       int64
@@ -337,6 +344,14 @@ func (a *Aggregator) refresh(ctx context.Context) {
 	if bi, err := a.RPC.GetBlockchainInfo(ctx); err == nil {
 		next.Chain = bi
 		next.BitcoinOK = true
+		// Track when the tip height last changed.
+		if bi.Blocks != a.lastTipHeight {
+			a.lastTipHeight = bi.Blocks
+			a.lastTipChangedAt = time.Now()
+		}
+		if !a.lastTipChangedAt.IsZero() {
+			next.TipChangedAge = time.Since(a.lastTipChangedAt).Seconds()
+		}
 		if nh, err := a.RPC.GetNetworkHashPS(ctx, -1, int(bi.Blocks)); err == nil {
 			next.NetworkHashrateHs = nh
 		}