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>

app/models/user.rb

@@ -29,6 +29,15 @@ class User < ApplicationRecord
   validates :first_name,  length: { minimum: 2, maximum: 50, allow_blank: true }
   validates :company_name, length: { minimum: 2, maximum: 100, allow_blank: true }
 
+  # Onboarding methods
+  def needs_onboarding?
+    !onboarding_completed?
+  end
+
+  def complete_onboarding!
+    update!(onboarding_completed: true)
+  end
+
   # Authorization methods
   def can_manage_events?
     # For now, all authenticated users can manage events