refactor: streamline routes to RESTful naming conventions

Remove /api/ prefix and consolidate route groups:
- /api/lobby/* -> /games, /snake, /logout (top-level)
- /game/{game_id} + /api/game/{game_id}/* -> /games/{id}/*
- /snake/{game_id} + /api/snake/{game_id}/* -> /snake/{id}/*
- /api/auth/* -> /auth/*
- Standardize snake join page to use return_url= (was return=)

features/lobby/handlers.go

@@ -91,7 +91,7 @@ func HandleCreateGame(store *game.GameStore, sessions *scs.SessionManager) http.
 
 		gi := store.Create()
 		sse := datastar.NewSSE(w, r)
-		sse.ExecuteScript(fmt.Sprintf("window.location.href='/game/%s'", gi.ID())) //nolint:errcheck
+		sse.ExecuteScript(fmt.Sprintf("window.location.href='/games/%s'", gi.ID())) //nolint:errcheck
 	}
 }