feat: Implement comprehensive onboarding system for new users

Add complete user onboarding flow that redirects new users to complete their
profile before accessing the application:

- Add onboarding_completed boolean field to users with migration
- Create OnboardingController with form validation and completion logic
- Design professional onboarding UI with progressive disclosure for company info
- Implement Stimulus controller for toggling company information section
- Add application-wide redirect middleware for incomplete users
- Create comprehensive test suite for all onboarding functionality
- Update test fixtures and helpers to support onboarding in existing tests

The onboarding collects required first/last name and optional company information.
Users are redirected to onboarding after login until profile is completed.
Features smooth animations, full-width form button, and clean UX design.

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

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

config/routes.rb

@@ -31,6 +31,10 @@ Rails.application.routes.draw do
     confirmation: "auth/confirmations"   # Custom controller for confirmations
   }
 
+  # === Onboarding ===
+  get "onboarding", to: "onboarding#index", as: "onboarding"
+  post "onboarding", to: "onboarding#complete", as: "complete_onboarding"
+
   # === Pages ===
   get "dashboard", to: "pages#dashboard", as: "dashboard"