🤖 fix: trim phantom blank line in highlighted code blocks

[ammar-agent]

Fixes a markdown rendering issue where a ```text code fence containing only a URL could render with an extra blank line.

• Treat visually-empty Shiki lines (e.g. <span></span>) as empty when extracting per-line HTML
• Add a unit test for extractShikiLines
• Extend the existing App/Markdown → CodeBlocks story with a play assertion to cover the regression

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