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/controllers/onboarding_controller.rb

@@ -0,0 +1,38 @@
+class OnboardingController < ApplicationController
+  before_action :authenticate_user!
+  before_action :redirect_if_onboarding_complete, except: [:complete]
+
+  def index
+    # Display the onboarding form
+  end
+
+  def complete
+    if onboarding_params_valid?
+      current_user.update!(onboarding_params)
+      current_user.complete_onboarding!
+      
+      flash[:notice] = "Bienvenue sur AperoNight ! Votre profil a été configuré avec succès."
+      redirect_to dashboard_path
+    else
+      flash.now[:alert] = "Veuillez remplir tous les champs requis."
+      render :index
+    end
+  end
+
+  private
+
+  def onboarding_params
+    params.require(:user).permit(:first_name, :last_name, :company_name)
+  end
+
+  def onboarding_params_valid?
+    onboarding_params[:first_name].present? && 
+    onboarding_params[:last_name].present?
+  end
+
+  def redirect_if_onboarding_complete
+    if current_user&.onboarding_completed?
+      redirect_to dashboard_path
+    end
+  end
+end