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>

test/fixtures/users.yml

@@ -5,9 +5,11 @@ one:
   encrypted_password: <%= Devise::Encryptor.digest(User, 'password123') %>
   last_name: Trump
   first_name: Donald
+  onboarding_completed: true
 
 two:
   email: user2@example.com
   encrypted_password: <%= Devise::Encryptor.digest(User, 'password123') %>
   last_name: Obama
   first_name: Barack
+  onboarding_completed: true