feat(homepage): limit upcoming events to 5, preserve 3 dates

[mroderick]

⚠️ In order to really replicate the challenge in this one locally, #2499 should be merged first (or at least used for seeding the development database locally)

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Summary

• Limit homepage to display max 5 upcoming events (workshops + meetings + events)
• Ensure at least 3 dates are shown even if fewer than 5 events
• Add conditional "Explore all events →" link that appears when more events exist

Changes

• dashboard_controller.rb: Added MAX_WORKSHOP_QUERY constant, @has_more_events instance variable, total_upcoming_events_count helper, and updated upcoming_events method to limit results
• show.html.haml: Added conditional link to /events

Testing

• All 857 tests passing

Manual Verification

1. Reseed the database (this takes a while):

   bundle exec rake db:drop db:create db:migrate db:seed
   

2. Start the server: bundle exec rails server
3. Visit http://localhost:3000
4. Scroll to the "Upcoming events" section
5. Verify:
   • No more than 5 events are displayed
   • At least 3 different dates are shown (if 3+ dates exist with events)
   • If there are more than 5 upcoming events, an "Explore all events →" button appears below the list
   • If there are 5 or fewer upcoming events, the button does not appear