Use image renditions for the editor's thumbnails

[ssavutu]

Why

The editor still draws every thumbnail from the original file. The library is mostly camera originals (often 5–80 MB), so the media grid downloads dozens of full-size photos to draw ~150px squares. The article list does the same for its 48px thumbnails. The API already sends resized renditions with these images (#239, #241), so the editor just needs to use them.

Change

• lib/imageVariants.ts: an ImageVariant type and srcSetFor(). It returns undefined when there are no renditions, so the attribute is omitted, never empty.
• srcSet + sizes on:
  • the media library grid (2–6 columns) and its detail preview (400px)
  • the media picker grid (2–5 columns)
  • the article list thumbnails (48px)
  • the featured-image preview in the editor (96px). Its renditions are stored with the URL they came with, because that URL can also be typed or pasted; a stale set is never shown against a different image. Only photo_url is saved, as before.
• src stays the original everywhere, so images the backfill hasn't reached look exactly as they do now.
• Not changed: images inserted into article bodies (trixImageHtml). Their URL is saved into the content and must stay the original. Readers get the renditions through content_image_variants (Return renditions for the images inside an article body #240).

Verified

• vitest: 23/23, including new tests for srcSetFor (including the empty and missing cases) and a picker test checking a tile keeps the original as src and gets the renditions as srcset.
• npm run build (tsc -b + vite) passes. eslint reports no new issues; the one exhaustive-deps warning in editArticleView.tsx is already on main.
• Not exercised in a browser: the editor needs an OIDC login, which I don't have locally.

🤖 Generated with Claude Code

https://claude.ai/code/session_019EcFyy8aqNnUd75CDPY6Bx