fix: normalize image reference paths to POSIX format

[sriharan0804]

Summary

This PR fixes Windows-specific image reference paths when exporting documents with referenced images.

Currently, image URIs are stored using the platform-specific path separator. On Windows this results in backslashes (\), which are URL-encoded as %5C in HTML output and also appear in Markdown image references.

This change normalizes the generated image URI to POSIX format using Path(...).as_posix(), ensuring consistent forward-slash (/) separators across platforms.

Changes

• Normalize referenced image paths using Path(obj_path).as_posix().
• Preserve cross-platform compatibility for HTML and Markdown image references.

Testing

• Verified the implementation on Windows.
• Confirmed that generated image references use / instead of \ (or %5C in HTML).
• Ran formatting and lint checks locally.

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