feat: introduce backend url environment variable

ref: N25B-352

README.md

@@ -28,6 +28,14 @@ npm run dev
 
 It should automatically reload when you save changes.
 
+## Environment
+
+Copy `.env.example` to `.env.local` and adjust values as needed:
+
+```shell
+cp .env.example .env.local
+```
+
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