JS-9749: image loader fixes for chat attachments and editor image block

[requilence]

Summary

• Chat image attachments now show the standard <Loader> while the gateway request is in flight, mirroring the editor image block pattern.
• Fixes a recently introduced regression in the editor image block (JS-9425) where the wrap collapsed to 0×0 during load on un-resized blocks: display: inline-block + only aspect-ratio (no width) cannot resolve dimensions, so the loader had no area to render in. Now the wrap is reserved with the natural widthInPixels (capped by max-width: 100%) when no user-set width is present.
• Loader overlay sits centered on a themed light-grey background (--color-shape-highlight-light) with rounded corners while loading, and vanishes the moment onLoad fires.

Retry/reload behavior on failed loads is intentionally out of scope and will land in a separate PR.

Test plan

• Send an image in chat — loader appears while gateway request is pending, image swaps in cleanly on load.
• Open a page with a freshly inserted (un-resized) image block — wrap reserves correct aspect-ratio space immediately, loader is centered on the light-grey background, image fills the same box on load with no jump.
• Open a page with a previously-resized image block — user width is preserved, no regression.
• Verify in dark mode — loader background uses the dark theme variant (rgba(255,255,255,0.03)).
• Verify chat multi-image grid layouts (2/3/4/5+ attachments) — loaders render correctly inside each cell.