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

@@ -1,7 +1,6 @@
 package c4game
 
 import (
-	"encoding/json"
 	"fmt"
 	"net/http"
 	"strconv"
@@ -18,6 +17,7 @@ import (
 	"github.com/ryanhamamura/games/db/repository"
 	"github.com/ryanhamamura/games/features/c4game/pages"
 	"github.com/ryanhamamura/games/sessions"
+	appsse "github.com/ryanhamamura/games/sse"
 )
 
 // c4ChatColors maps player color (1=Red, 2=Yellow) to CSS background colors.
@@ -120,10 +120,7 @@ func HandleGameEvents(store *connect4.Store, nc *nats.Conn, sm *scs.SessionManag
 		}
 
 		sendPing := func() error {
-			data, _ := json.Marshal(struct {
-				LastPing int64 `json:"lastPing"`
-			}{time.Now().UnixMilli()})
-			return sse.PatchSignals(data)
+			return appsse.SendPing(sse)
 		}
 
 		// Send initial render and ping

features/snakegame/handlers.go

@@ -1,7 +1,6 @@
 package snakegame
 
 import (
-	"encoding/json"
 	"fmt"
 	"net/http"
 	"strconv"
@@ -17,6 +16,7 @@ import (
 	"github.com/ryanhamamura/games/features/snakegame/pages"
 	"github.com/ryanhamamura/games/sessions"
 	"github.com/ryanhamamura/games/snake"
+	appsse "github.com/ryanhamamura/games/sse"
 )
 
 func snakeChatColor(slot int) string {
@@ -126,10 +126,7 @@ func HandleSnakeEvents(snakeStore *snake.SnakeStore, nc *nats.Conn, sm *scs.Sess
 		}
 
 		sendPing := func() error {
-			data, _ := json.Marshal(struct {
-				LastPing int64 `json:"lastPing"`
-			}{time.Now().UnixMilli()})
-			return sse.PatchSignals(data)
+			return appsse.SendPing(sse)
 		}
 
 		// Send initial render and ping

sse/signals.go

@@ -0,0 +1,28 @@
+// Package sse provides helpers for SSE signal handling.
+package sse
+
+import (
+	"encoding/json"
+	"time"
+
+	"github.com/starfederation/datastar-go/datastar"
+)
+
+// Signals holds client-side state managed via SSE.
+type Signals struct {
+	LastPing int64 `json:"lastPing,omitempty"`
+}
+
+// SendPing sends a heartbeat signal with the current timestamp.
+func SendPing(sse *datastar.ServerSentEventGenerator) error {
+	return PatchSignals(sse, Signals{LastPing: time.Now().UnixMilli()})
+}
+
+// PatchSignals sends a signals patch to the client.
+func PatchSignals(sse *datastar.ServerSentEventGenerator, s Signals) error {
+	data, err := json.Marshal(s)
+	if err != nil {
+		return err
+	}
+	return sse.PatchSignals(data)
+}