fix(publisher): draw media backgrounds at the original's size under background-size auto

[tommy230]

Summary

A media-library image used as a background with background-size: auto (or no size set) publishes at about 1024px wide, whatever its real size. A 400px decorative shape covers most of its section.

.card {
  background-image: url('/uploads/shape.png'); /* 400px wide */
}

The image-set() descriptors are relative to a fixed 1024px reference, so the 400px variant gets 0.39x. Under auto the browser draws it at 400 / 0.39 ≈ 1024 CSS px.

• When every background-size layer is auto (or the property is unset), descriptors are now relative to the asset's recorded width, so the drawn size matches the original.
• Cover, contain and sized backgrounds are unchanged. They keep the 1024px reference and the same descriptors as today, so a DPR-1 screen still downloads the same variant.
• 1024px stays as the fallback when an asset has no recorded width.

Verification

• bun run build
• bun run lint
• bun test - 7063 pass, 0 fail
• Docker/deployment check, if relevant - not relevant

The new auto / unset tests in src/__tests__/publisher/classStyleInjector.test.ts fail on main and pass with this change. The new cover/contain/sized tests pass on both. The existing descriptor tests keep their expected values and now set background-size: cover.

Checklist

• Tests cover behavior changes.
• Docs were updated when behavior, config, deployment, or public surfaces changed. One sentence in docs/features/publisher.md.
• No compatibility shim was added for old pre-release behavior.
• No secrets, local databases, uploads, or generated artifacts are included.

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