fix: preserve percentage image widths after reload

[Found-L]

Summary

• Preserve percentage-based image widths when parsing HTML.
• Apply the same width parsing behavior to inline and block image nodes.
• Keep the existing behavior for pixel and numeric widths unchanged.

Problem

Image widths such as 100% were parsed with Number.parseInt, which converted them to 100. After reloading the document, this numeric value was rendered as 100px, causing percentage-sized images to shrink unexpectedly.

Changes

Added a shared image width parser that:

• preserves percentage widths as strings, such as 100%
• parses pixel and numeric widths as numbers, such as 500px to 500
• returns null when no width is provided

The parser is now used by all HTML parsing paths for both Image and ImageBlock.

Validation

• Verified that 100% remains 100% for inline and block images after reloading.
• Verified that 500px continues to parse as 500.
• Passed pnpm type-check.
• Passed pnpm build:lib.
• Passed git diff --check.

Fixes #147

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