fix: make public QR viewer self-contained

[reloxx13]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "x" next to an item)

• Documentation update
• Bug fix
• New feature
• Other, please explain:

What changes did you make? (Give an overview)

This fixes the public QR viewer when the host only exposes view.php and api/download.php, but not the usual Photobooth asset paths.

• make view.php self-contained so it no longer depends on shared CSS/JS/font assets that 404 on the public host
• load the preview image through api/download.php?inline=1 instead of a direct data/images/... URL
• extend api/download.php to support inline image/video responses while keeping normal download behavior unchanged
• use the configured event text when available and fall back to the start screen title if not

Is there anything you'd like reviewers to focus on?

Please focus on whether the standalone view.php approach is acceptable for public QR endpoints where the full asset tree is not reachable.

AI used to create this Pull Request?

AI was used to implement and describe the changes. The final diff was reviewed in the local checkout and validated with:

• ddev php -l /var/www/html/view.php
• ddev php -l /var/www/html/api/download.php