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/pages/game.templ

@@ -15,9 +15,10 @@ templ GamePage(g *connect4.Game, myColor int, messages []chat.Message, chatCfg c
 	@layouts.Base("Connect 4") {
 		<main
 			class="flex flex-col items-center gap-4 p-4"
-			data-signals="{chatMsg: ''}"
+			data-signals="{chatMsg: '', lastPing: 0}"
 			data-init={ fmt.Sprintf("@get('/games/%s/events',{requestCancellation:'disabled'})", g.ID) }
 		>
+			@sharedcomponents.ConnectionIndicator()
 			@GameContent(g, myColor, messages, chatCfg)
 		</main>
 	}