Switch Twitter embeds to iframe-only approach

Replace TwitterPlugin (which loads external widgets.js script) with
TwitterEmbedder that generates iframes directly.

Changes:
- Add TwitterEmbedder extending AbstractEmbedder
- Register TwitterEmbedder in AssetEmbedder
- Remove TwitterPlugin from blog renderer plugins
- Add test coverage

Benefits:
- No external scripts loaded (privacy improvement)
- Consistent with YouTube/Vimeo/3speak embedders
- Simpler CSP: only frame-src needed, not script-src
- Fixes eager script loading issue for Twitter
- Works with both twitter.com and x.com URLs (rebrand)

The embed iframe uses platform.twitter.com which remains stable
regardless of the x.com rebrand.

Relates to #786