Smoother article-update flow, no more flicker between steps

The article-update flow had a few rough edges that added up to a frustrating experience when polishing a long post.

The biggest change: the four edit steps — basic, content, metadata, publish — now share a single persistent shell. The stepper, header, and form context no longer remount as you click between steps, which means no more flicker, no more re-fetching, no more losing scroll position.

Other fixes shipped in the same release:

• The block drag handle in the editor now works correctly for images, dividers, and embedded videos. Before, the gutter handle silently no-op'd on those nodes.
• Change History modal got several papercut fixes: long URLs and HTML diffs now wrap properly, view-count entries no longer pollute the timeline with empty diffs, and content changes render as a friendly i18n string instead of a truncated HTML snippet.
• The dashboard navigation lost the Settings tab — still reachable from the avatar dropdown, but no longer cluttering the primary nav.

If you spend serious time in the article editor, this release reclaims a lot of small frictions all at once.