feat: add connection status indicator with SSE heartbeat

- Add ConnectionIndicator component showing green/red dot
- Send lastPing signal every 15 seconds via SSE
- Indicator turns red if no ping received in 20 seconds
- Gives users confidence the live connection is active

features/c4game/handlers.go

@@ -1,6 +1,7 @@
 package c4game
 
 import (
+	"encoding/json"
 	"fmt"
 	"net/http"
 	"strconv"
@@ -118,11 +119,22 @@ func HandleGameEvents(store *connect4.Store, nc *nats.Conn, sm *scs.SessionManag
 			return sse.PatchElementTempl(pages.GameContent(g, myColor, room.Messages(), chatCfg))
 		}
 
-		// Send initial render
+		sendPing := func() error {
+			ping, _ := json.Marshal(map[string]int64{"lastPing": time.Now().UnixMilli()})
+			return sse.PatchSignals(ping)
+		}
+
+		// Send initial render and ping
+		if err := sendPing(); err != nil {
+			return
+		}
 		if err := patchAll(); err != nil {
 			return
 		}
 
+		heartbeat := time.NewTicker(15 * time.Second)
+		defer heartbeat.Stop()
+
 		// Subscribe to game state updates
 		gameCh := make(chan *nats.Msg, 64)
 		gameSub, err := nc.ChanSubscribe(connect4.GameSubject(gameID), gameCh)
@@ -140,6 +152,10 @@ func HandleGameEvents(store *connect4.Store, nc *nats.Conn, sm *scs.SessionManag
 			select {
 			case <-ctx.Done():
 				return
+			case <-heartbeat.C:
+				if err := sendPing(); err != nil {
+					return
+				}
 			case <-gameCh:
 				if err := patchAll(); err != nil {
 					return