Add Varbase Events Base and render the event listings from the events view

Problem/Motivation

The site template has no Event content type. The Events page renders the blog view, and the Home page shows three hand-placed event cards that all read "Digital Security for Advocacy Groups". A site builder cannot add an event, and nothing about an event can be listed, filtered or dated.

Steps to reproduce

1. Install the Horizon Aid site template.
2. Open /admin/content and try to add an event: there is no Event content type.
3. Open /events: the listing is the blog view, not events.
4. Open the Home page: three identical hand-placed event cards.

Proposed resolution

• Compose varbase_events_base into the site template, so the Event content type, its fields (when, location, categories, description, featured image), the events view and the Drupal Canvas templates all come from the base recipe rather than being redefined here.
• Point the Events page and the Home "Upcoming Events" section at the events view, rendered through its card view mode, and remove the hand-placed cards.
• Repoint the events view and the event Canvas templates to the site theme with the Varbase Recipes config actions, so they do not render through the base theme.
• Add three demo events with their categories, dates and locations: a webinar, a policy briefing and a summit.

Remaining tasks

• ✅ File an issue about this project
• ❌ Addition/Change/Update/Fix to this project
• ❌ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ❌ Update Release Notes
• ❌ Release

User interface changes

• An Event content type is available, and events are published under /events/.
• The Events page and the Home "Upcoming Events" section follow the content.

API changes

• N/A

Data model changes

• The Event content type and its fields come from varbase_events_base, together with an Event categories vocabulary.

Release notes snippet

• Added Varbase Events Base with three demo events, and moved the event listings onto the events view.