Add Rails engine with browser UI at /whetrails

Mounts a Rails engine that lists app models in a sidebar and renders
the full callback/validator chain for any model. Source locations are
linked via vscode:// URIs. Condition badges and concern module tags
distinguish at-a-glance where each callback comes from.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/whetrails.rb

@@ -1,6 +1,7 @@
 require_relative "whetrails/version"
 require_relative "whetrails/chain"
 require_relative "whetrails/inspector"
+require_relative "whetrails/engine" if defined?(Rails)
 
 module Whetrails
   def self.inspect_model(model_class)