Fix import.meta.env.MODE being undefined in host apps

.env.development

@@ -1,3 +1,5 @@
+MODE="development"
+
 VITE_APP_BACKEND_V2_GET_URL=https://json-dev.excalidraw.com/api/v2/
 VITE_APP_BACKEND_V2_POST_URL=https://json-dev.excalidraw.com/api/v2/post/
 

.env.production

@@ -1,3 +1,5 @@
+MODE="production"
+
 VITE_APP_BACKEND_V2_GET_URL=https://json.excalidraw.com/api/v2/
 VITE_APP_BACKEND_V2_POST_URL=https://json.excalidraw.com/api/v2/post/