Improve Event Attendees Index with event-specific title and hidden event column

[Copilot]

This PR enhances the Event Attendees Index page to provide a more focused view when filtering by a specific event.

Changes Made

Controller Improvements

• Added performance optimization by including profile data in the query when event_id is present
• Set @event instance variable when event_id parameter is provided using a before_action callback
• Maintained clean, readable code structure with proper separation of concerns

View Enhancements

When an event_id parameter is provided:

• Title: Changes from "Event Attendees" to "#{event.name} Attendees" (e.g., "RubyConf 2024 Attendees")
• Table: Hides the "Event" column since all attendees are for the same event

When no event_id is provided, the page maintains its original behavior showing all user's event attendances with both Profile and Event columns.

Testing

• Added comprehensive feature tests in spec/features/event_attendees/index_spec.rb
• Tests verify title changes and column visibility for both scenarios
• All existing tests continue to pass

UI Impact

The changes provide a cleaner, more focused interface when viewing attendees for a specific event while maintaining full backward compatibility for the general attendees list.

Before (default view):

• Title: "Event Attendees"
• Columns: Profile | Event

After (with event_id):

• Title: "RubyConf 2024 Attendees"
• Columns: Profile only

Fixes #312.

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