fix: add env_file to backend-dev service

Backend needs access to .env file for environment variables like
COOKIE_SECRET and HANKO_API_URL.

🤖 Generated with Claude Code

Author: hg1g

compose.yaml

@@ -264,6 +264,7 @@ services:
 
   # custom login backend dev
   backend-dev:
+    env_file: .env
     build:
       context: ./backend
       dockerfile: Dockerfile