Add email display to person profile buttons and organization dropdowns

[Copilot]

What is the goal of this PR and why is this important?

Person identification requires email visibility when multiple people share similar names. This adds email display in two contexts: person profile buttons (organizations index → people index) and person selection dropdowns (organization edit form).

How did you approach the change?

Added Person#preferred_email method

• Centralizes email prioritization: user.email → person.email → email_2
• Reusable across views and controllers

Updated people index view

<%= person_profile_button(person, subtitle: person.preferred_email) %>

Updated organization form dropdown

• Changed joins(:user) → left_joins(:user) to include people without user accounts
• Uses pluck with COALESCE SQL for performance (avoids loading full records)
• Format: "First Last (email@example.com)"

@people_array = Person.left_joins(:user)
  .pluck(:first_name, :last_name, :id, 
         Arel.sql("COALESCE(users.email, people.email, people.email_2)"))
  .map { |fn, ln, id, email| ["#{fn} #{ln}#{" (#{email})" if email.present?}", id] }

Anything else to add?

User associations already preloaded in people controller (line 10), avoiding N+1 queries.

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