Refactor to latest Nextra 4 patterns

[torn4dom4n]

This PR refactors the Fancy Notes project to align with the latest Nextra 4 and Next.js 15+ patterns.

Key changes:

1. Catch-all Route Refactor: Converted app/[[...mdxPath]]/page.jsx to page.tsx. Added proper TypeScript types and improved generateMetadata to use page-specific titles for dynamic OG image generation.
2. Global MDX Components: Updated mdx-components.tsx to include Callout and Steps from nextra/components. This allows using these components in MDX files without explicit imports.
3. Sidebar Management: Added content/_meta.js to define the sidebar structure, ordering, and human-readable titles for the notes, replacing the default file-based naming.
4. Layout Simplification: Updated app/layout.tsx to follow the latest Nextra 4 Layout pattern and simplified global metadata.
5. MDX Cleanup: Removed redundant Steps and Callout imports from all existing MDX files.

Verified the changes with pnpm run build and a Playwright verification script confirming that the sidebar titles are correctly applied and the site builds successfully.

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PR created automatically by Jules for task 10764288598827751889 started by @torn4dom4n

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[reactreview]

Copy as prompt

Check if these React Review issues are valid. If so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.

Run this before and after your changes to verify the result:
npx react-doctor@latest --verbose --diff

Do not modify the react-doctor configuration unless explicitly asked.
Fix the underlying code issues instead of changing or suppressing the rules.

React Review found 0 errors and 1 warning. This PR leaves the React health score unchanged.

<file name="content/_meta.js">

<violation number="1" location="content/_meta.js:0">
Severity: Warning

Unused file — not reachable from any entry point

Delete the file if it is truly unreachable, or import it from an entry point.

Rule: `unused-file`
</violation>

</file>

^{Reviewed by reactreview for commit 0df6879. Configure here.}