fix: remove container border/bg from image previews in chat

[wesbillman]

Summary

Removes the visible container box (border, background, flex centering) around image preview thumbnails in chat messages. Images now render cleanly with rounded corners applied directly to the <img> element.

Changes

File: desktop/src/shared/ui/markdown.tsx (3 hunks)

1. Image preview wrapper — removed flex items-center justify-center overflow-hidden rounded-2xl border border-border/70 bg-muted/40, tightened mt-3 → mt-1
2. Image element — added rounded-xl for corner rounding directly on the image (replaces container-level overflow-hidden clipping)
3. Multi-image gallery grid — mt-3 → mt-1 for consistent spacing

What's preserved

• Lightbox click-to-expand behavior (DOM structure untouched)
• Video preview path (completely separate code path)
• Gallery grid layout ([&_div]:mt-0 [&_div]:max-w-none overrides still target correctly)

NIT (design awareness)

Transparent PNGs will now show the chat background directly instead of the old bg-muted/40 fill. This is the intended behavior — removing the container means no artificial background behind images.

Before / After

Before: Images wrapped in a bordered, background-filled container
After: Images render directly with rounded corners, no visible container

[wesbillman]

BEFORE:

AFTER: