fix: match MultimodalInput background to artifact panel

[khoinguyenpham04]

Summary

• Fixes a visual bug where the MultimodalInput wrapper in the artifact panel shows a sharp-cornered
  rectangle behind the rounded input form in both light and dark mode

Problem

The artifact panel (motion.div, line 303) uses bg-muted dark:bg-background, but the
MultimodalInput wrapper (line 340) uses the inverse: bg-background dark:bg-muted.

Since the wrapper div has no border-radius, it's visible as a sharp rectangle behind the form's
rounded-xl corners:

• Light mode: white rectangle against the muted gray panel
• Dark mode: lighter gray rectangle against the dark panel

Screenshots

Before (bug)

After (fix)

Fix

Align the MultimodalInput className to match the parent panel's background:

- className="bg-background dark:bg-muted"
+ className="bg-muted dark:bg-background"

Test plan

- Open an artifact (e.g. code generation) so the side panel appears
- Verify no sharp-cornered rectangle around the chat input in light mode
- Switch to dark mode and verify the same

Removed the `alt` and `height` attributes — sometimes those cause rendering issues on GitHub. Just
`src` and `width` is the most reliable.

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