feat(cluster): Chase New — a panel for what is new and audible here

PSK Reporter tells you what is actually being decoded in your region, which
is a larger set than what somebody chose to spot: nobody spots the FT8 caller
running ten watts from a rare square.

Almost all of it existed. The MQTT payload already carries frequency, mode,
transmitter and both grids; the watcher already drops any report collected
further than NearKm from the operator, which is exactly the question worth
asking — the station is being heard HERE, not in Japan; and with grid chasing
on the subscription is already every band, filtered at the broker by receiver
square, measured at 0.2 to 1.2 messages a second. This reads messages that
were arriving and being discarded.

"New" is not decided here. Every spot goes through ClusterSpotStatuses, the
same function the DX cluster grid uses and the same cached index, so the two
panels cannot drift apart the way the county columns did. Cost per message is
map lookups behind an option cached in an atomic, because the MQTT goroutine
must never wait on the settings store.

The option is its own, not nested under grid chasing: chasing squares and
chasing entities are different wants, and the feed now has three consumers,
any one of which brings it up and none of which cuts the others loose when it
goes down.

The panel says "digital modes only" in its footer. An empty list has to mean
"nothing new on FT8/FT4/JS8 near you", not "the band is dead" — it will never
show a new entity on CW.

app.go

@@ -650,6 +650,12 @@ type App struct {
 	// bounds itself by age instead, so memory follows how many distinct stations
 	// have actually been heard in two years rather than a made-up ceiling.
 	gridStore *gridcache.Store
+
+	// chaseNew holds the "new against the log" stations PSK Reporter is hearing
+	// near here; chaseNewOn is the option cached for the MQTT goroutine, which
+	// consults it once per message and must not reach the settings store.
+	chaseNew   *chaseNewStore
+	chaseNewOn atomic.Bool
 	// pskr is the PSK Reporter MQTT feed, up only while the opening watch is on.
 	// It is the source that makes VHF detection work at all: the cluster and RBN
 	// carry a handful of 6 m spots where PSK Reporter carries hundreds.
@@ -1415,8 +1421,11 @@ func (a *App) startup(ctx context.Context) {
 	go a.chatLoop()        // multi-op: poll the shared chat + heartbeat presence
 	go a.hrdlogOnAirLoop() // publish frequency/mode/rig on hrdlog.net when enabled
 	// Locator store BEFORE the feed: the feed asks whether it exists to decide
-	// which bands to subscribe to.
+	// which bands to subscribe to. Same for the chase-new option, read into its
+	// atomic here so the feed and the MQTT goroutine agree from the first message.
 	a.startGridCache()
+	a.chaseNew = newChaseNewStore()
+	a.refreshChaseNew()
 	// PSK Reporter. After the operator's grid is known: without it there is no
 	// distance to measure and no receiver squares to filter on, so it stays down.
 	a.startBandOpenFeed()