Bug: Validate image style names before building derivative URLs

Problem/Motivation

Image structure builders accept a $styleName argument and pass it straight through (for example the blog_teaser default). If the style does not exist on the site, the code either fails or silently returns the original file URL, which can serve full-size images to the frontend.

Proposed resolution

• Load the image style through the entity type manager and verify it exists before generating a URL.
• Log a warning and fall back to the original URL deliberately rather than by accident.
• Make the default style names configurable rather than hardcoded.

Remaining tasks

Audit every image-building method and add the validation.

User interface changes

Configurable default image styles.

API changes

None.