Change frontend .env files to be committed and have .env.*.local overrides (#6207)

.gitignore

@@ -163,9 +163,6 @@ __pycache__/
 *.pyo
 
 # Virtual environments
-.env*
-!.env*.example
-!engine/.env
 .venv*
 env*/
 venv*/
@@ -173,6 +170,9 @@ ENV/
 env.bak/
 venv.bak/
 
+# Env files (secrets / local overrides). Subproject .gitignore files whitelist any committed defaults.
+.env*
+
 # VS Code
 /.vscode/**/*
 !/.vscode/settings.json