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"
@@ -119,7 +120,10 @@ func HandleGameEvents(store *connect4.Store, nc *nats.Conn, sm *scs.SessionManag
 		}
 
 		sendPing := func() error {
-			return sse.PatchSignals([]byte(fmt.Sprintf(`{"lastPing":%d}`, time.Now().UnixMilli())))
+			data, _ := json.Marshal(struct {
+				LastPing int64 `json:"lastPing"`
+			}{time.Now().UnixMilli()})
+			return sse.PatchSignals(data)
 		}
 
 		// Send initial render and ping

features/snakegame/handlers.go

@@ -1,6 +1,7 @@
 package snakegame
 
 import (
+	"encoding/json"
 	"fmt"
 	"net/http"
 	"strconv"
@@ -125,7 +126,10 @@ func HandleSnakeEvents(snakeStore *snake.SnakeStore, nc *nats.Conn, sm *scs.Sess
 		}
 
 		sendPing := func() error {
-			return sse.PatchSignals([]byte(fmt.Sprintf(`{"lastPing":%d}`, time.Now().UnixMilli())))
+			data, _ := json.Marshal(struct {
+				LastPing int64 `json:"lastPing"`
+			}{time.Now().UnixMilli()})
+			return sse.PatchSignals(data)
 		}
 
 		// Send initial render and ping