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/snakegame/pages/game.templ

@@ -32,11 +32,12 @@ templ GamePage(sg *snake.SnakeGame, mySlot int, messages []chat.Message, chatCfg
 	@layouts.Base("Snake") {
 		<main
 			class="snake-wrapper flex flex-col items-center gap-4 p-4"
-			data-signals={ `{"chatMsg":""}` }
+			data-signals={ `{"chatMsg":"","lastPing":0}` }
 			data-init={ fmt.Sprintf("@get('/snake/%s/events',{requestCancellation:'disabled'})", gameID) }
 			data-on:keydown__throttle.100ms={ keydownScript(gameID) }
 			tabindex="0"
 		>
+			@components.ConnectionIndicator()
 			@GameContent(sg, mySlot, messages, chatCfg, gameID)
 		</main>
 	}