fix: #3610873 Install Vartheme BS5, which the recipe's Canvas templates render through

What

The event page Canvas template (canvas.content_template.node.event.full) references Vartheme BS5 components (sdc.vartheme_bs5.section, .heading, .share, .date, .card-text, .button, ...). Config actions are validated on apply, so if the theme is not installed the recipe aborts the whole site install:

The 'canvas.component.sdc.vartheme_bs5.section' config does not exist.

This surfaced as soon as the recipe was applied from a site template that installs its own theme (Varbase Educare) instead of Vartheme BS5 directly.

Why installing the theme in the same recipe was not enough on its own

Drupal Canvas only generates a theme's Component config entities on RecipeAppliedEvent — after a recipe finishes applying, not mid-apply. So a recipe cannot install a theme and consume that theme's generated components inside the same config.actions pass; the components simply do not exist yet when the action runs.

Fix

Declare vartheme_bs5 in install, so it is installed (and its Canvas components generated) by a recipe that finishes before this recipe's own config.actions run — i.e. this recipe must be applied after whatever installs Vartheme BS5 in the composing site template. Varbase News Base declares the same install line for the same reason.

Issue

Closes #3610873: https://www.drupal.org/project/varbase_events_base/issues/3610873

AI disclosure

AI-Generated: Yes (Added vartheme_bs5 to the recipe's install list and wrote this MR description; reviewed by Rajab Natshah.)

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