[xExtension-YouTube] Add optional responsive player sizing

[JamBalaya56562]

Summary

• add an opt-in responsive sizing setting for the YouTube and PeerTube players
• preserve the existing youtube-plugin-video class and fixed-size behavior by default
• move player sizing out of the inline style attribute so it works with FreshRSS CSP
• keep the referrerpolicy introduced in [xExtension-YouTube] Add referrer policy to iframe in extension.php #382

This takes over the work from #291 and applies its review feedback to the current implementation.

Behavior

When automatic sizing is enabled, the player uses the existing youtube-plugin-video class together with yt-player-autosize, which applies a full-width 16:9 aspect ratio. When it is disabled or unset, the configured width and height attributes are preserved.

Validation

• npm run eslint
• npx stylelint "**/*.css"
• npm run markdownlint
• reviewed the generated iframe branches for YouTube, youtube-nocookie, PeerTube, and original-content handling

PHPStan and PHPCS were not run locally because PHP and make are unavailable on PATH and Docker Desktop could not start. They are expected to run in GitHub Actions.