fix: remove hardcoded CORS_ORIGINS from docker-compose, improve login errors

CORS_ORIGINS was hardcoded in docker-compose.yml, silently overriding .env
and breaking login for users who change the frontend port. It now comes
from .env exclusively, with a clear comment in .env.example.

Login page now distinguishes network errors (CORS/offline) from wrong
credentials, and footer correctly references .env instead of config.yml.

.env.example

@@ -1,6 +1,7 @@
 # Backend - server-side only (NEVER commit .env)
 SECRET_KEY=change_me_in_production
 SQLITE_PATH=./data/homelab.db
+# Set this to the URL(s) you use to access Homelable in your browser.
 CORS_ORIGINS=["http://localhost:5173","http://localhost:3000"]
 
 # Auth — default credentials: admin / admin