In the attachment preview modal, image and video previews used
`max-h-full max-w-full`, which let small assets render at their
natural size and leave the modal mostly empty. Switch to
`h-full w-full` so the preview always occupies the modal viewport,
relying on `object-contain` to preserve aspect ratio without
upscaling beyond the intrinsic bounds.
Only touches `packages/views/editor/attachment-preview-modal.tsx`
for the image (line 355) and video (line 373) branches; pdf, audio,
markdown, html, and text branches keep their existing layout.
Co-authored-by: multica-agent <github@multica.ai>