feat: #3614494 Fill the event banner from its fields: category, date and registration link

Issue: https://www.drupal.org/project/horizonaid/issues/3614494

AI-Generated: Yes

Problem

The event full page's banner carried no category, no date and no registration link — nothing on it came from the event's own fields. Two things blocked that:

• A Canvas binding that reaches a taxonomy term rendered empty for site visitors. It needs two permissions, checked in order: view terms in <vocabulary> (entity access) and view term names in <vocabulary> (field access). Granting only the first still fails. Varbase 11 grants neither to anonymous or authenticated, so every term-backed component is silently empty on a fresh site, and it surfaces as a Twig "NULL value found" error that reads like a broken expression.
• There was no field for a registration link at all.

Resolution

• New field_registration_url link field on the Event bundle (cardinality 1, label "Registration link"), plus its widget on the event form display.
• canvas.content_template.node.event.full: the banner is now, in order, a taxonomy chip bound to field_event_categories delta 0 → term name, the title, the description, a date bound to field_when delta 0 (Smart Date timestamp) with a calendar glyph, and a Register Today button bound to field_registration_url, with spacer components for the vertical rhythm and the copy kept in the 67-33 column band the design uses.
• grantPermissions on user.role.anonymous and user.role.authenticated for view terms in and view term names in on both event_categories and event_topics.

Multi-value fields do bind in a full content template as long as the expression carries an explicit delta (␞0␟) — an earlier assumption that they cannot was wrong. The real cause was the term permissions above.

Verification

Applied on a Varbase 11 dev site and checked in a real browser on two event pages: the chip reads "WEBINAR" and "In-person" respectively, each page shows its own date ("12 June 2027", "17 June 2027") with the calendar glyph, and the Register Today button renders as a pill opening in a new tab. 22 components in the template, config.typed validation reports 0 violations, 0 render errors on the page.

Dependencies and known risk

• The template pins the Canvas component_version of the Date component that the matching vartheme_bs5_horizonaid change introduces (issue #3614098), so this MR must land after that one.
• Separately, pinning component_version hashes at all currently breaks a fresh install of the published site template: applying canvas.content_template.node.event.full throws '64089b7681474537' is not a version that exists on component config entity 'sdc.vartheme_bs5_horizonaid.button'. Available versions: 'e151432379f663fb'. Both hashes exist on a finished site, so it is an ordering problem — at recipe-apply time during install the theme has registered only one version. That is a separate defect, not introduced here, and it is not fixed by this MR.

Checkpoints

• 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