fix(ui): don't refetch the lobby on heartbeats

The lobby refetched /user/games on every stream event, including the 10s
keep-alive heartbeat, turning it into a 10s poll. That poll's REST view of a
just-committed opponent move could flip a card (and now blink it) seconds before
the matching your_turn event — and its toast — arrived over the slower live
stream, so the new lobby-card blink appeared to lead the toast by 5-7s (variable
with stream delivery; in sync when prompt).

Gate the refetch to real events (kind !== 'heartbeat'): the card, its blink and
the toast now ride the same event (opponent_moved + your_turn are published
together), and the constant 10s full-lobby poll per client is gone.

ui/src/screens/Lobby.svelte

@@ -55,7 +55,11 @@
     void load();
   });
   $effect(() => {
-    if (app.lastEvent) void load();
+    // Refetch on a real game event only — not the 10 s keep-alive heartbeat. Refetching on every
+    // heartbeat turned the lobby into a 10 s poll whose REST view of a just-committed opponent move
+    // could update (and blink) a card seconds before the matching your_turn event/toast arrived
+    // over the slower live stream; gating it keeps the card, its blink and the toast on one event.
+    if (app.lastEvent && app.lastEvent.kind !== 'heartbeat') void load();
   });
 
   const myId = $derived(app.session?.userId ?? '');