feat: make invite link base URL configurable via APP_URL

Load environment variables from .env file using godotenv.
Defaults to https://demo.adriatica.io if APP_URL is not set.

ui/snakestatus.go

@@ -130,7 +130,7 @@ func SnakePlayerList(sg *snake.SnakeGame, mySlot int) h.H {
 }
 
 func SnakeInviteLink(gameID string) h.H {
-	fullURL := baseURL + "/snake/" + gameID
+	fullURL := getBaseURL() + "/snake/" + gameID
 	return h.Div(h.Class("mt-4 text-center"),
 		h.P(h.Text("Share this link to invite players:")),
 		h.Div(h.Class("bg-base-200 p-4 rounded-lg font-mono break-all my-2"),