Fix Copy Markdown button path resolution

[Copilot]

The Copy Markdown button fails with "Page not found" because path matching doesn't account for file extensions.

Issue

page.path from the component includes extensions (model/mac/biba.mdx), but source.getPages() returns paths without them (model/mac/biba), causing lookup to fail.

Changes

• Strip .mdx/.md extensions during path normalization in getMarkdownContent()

// Before matching against fumadocs pages
normalizedPath = normalizedPath.replace(/\.mdx?$/, "");

This aligns the incoming path format with fumadocs' internal representation before the lookup.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[bug] "Copy Markdown" button doesn't work</issue_title>
<issue_description>in docs page like: https://www.casbin.org/docs/biba, "Copy Markdown" button doesn't work, it shows error:

Failed to copy the markdown to your clipboard. Please copy it manually.

need to fix</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [bug] "Copy Markdown" button doesn't work #55

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