feat: add image control options (disable download/hover on images)

[sleitor]

Summary

Adds image to the controls prop, allowing users to disable the hover overlay and download button on images.

Closes #511

Usage

// Disable all image controls (hover overlay + download)
<Streamdown controls={{ image: false }}>{content}</Streamdown>

// Disable only the download button (keeps hover overlay)
<Streamdown controls={{ image: { download: false } }}>{content}</Streamdown>

Changes

• Extended ControlsConfig type to include image?: boolean | { download?: boolean }
• Added shouldShowImageControl helper following the existing mermaid/code/table pattern
• Updated MemoImg wrapper to read controls from context and pass props to ImageComponent
• Updated ImageComponent to accept showControls and showDownloadControl props
• Added test coverage for all new control combinations
• Added changeset (minor bump)

[vercel]

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