Add "Event" to all chapter pages

[Agboolafeyikemi]

Description

Adds an "Add to all" option at the top of the Chapters dropdown on the admin event form, so organisers can invite every chapter with one click instead of ticking each chapter individually.

This implements option 2 from the discussion on the issue (the in-dropdown "Add to all" entry), which @KimberleyCook preferred.

Closes #2490

Changes

• New add-all-chapters.js: injects a bold "Add to all" row at the top of the Chosen dropdown for the chapters field. Clicking it selects every chapter, refreshes the widget, and closes the dropdown.
• Small style block in main.scss for the new row (bold, pointer cursor, hover highlight).
• Playwright feature spec covering the flow end-to-end: open the dropdown, click "Add to all", verify the dropdown closes, save, and assert the event is linked to every chapter.

How to test

1. Sign in as an admin and open /admin/events/new (or edit an existing event).
2. Scroll to Invitation details → Chapters and click into the field.
3. Click the bold Add to all entry at the top of the dropdown.
4. All chapters are selected and the dropdown closes.

Or run the spec:

bundle exec rspec spec/features/admin/manage_event_spec.rb