Add Forest Admin configuration with environment variables

Use environment variables for auth_secret and env_secret to ensure secrets are not committed to the repository.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: PMerlet

config/initializers/forest_admin_rails.rb

@@ -0,0 +1,4 @@
+ForestAdminRails.configure do |config|
+  config.auth_secret = ENV['FOREST_AUTH_SECRET']
+  config.env_secret = ENV['FOREST_ENV_SECRET']
+end