feat: enhance internal link localization for blog posts and standard pages

[danielmarv]

This pull request introduces a locale-aware system for internal links in markdown-rendered content. Authors now write internal links without a locale prefix, and the renderer automatically adds the correct locale prefix based on the current page's locale. This ensures that links resolve correctly for both English and German content, prevents double-prefixing, and leaves external links, anchors, and asset paths unchanged.

Locale-aware internal link handling:

• Added the localizeInternalLinks function in src/lib/markdown.ts to rewrite locale-neutral internal links so they resolve within the rendered locale, using the correct /de/ or /en/ prefix as needed.
• Updated the markdown processing pipeline to apply localizeInternalLinks after HTML conversion, ensuring all internal links in page content are correctly localized (getPostBySlug in src/lib/markdown.ts, src/lib/markdown.tsR787-R790; loadPageData in src/app/[locale]/[...rest]/page.tsx, src/app/[locale]/[...rest]/page.tsxL98-R99; import added, src/app/[locale]/[...rest]/page.tsxR9)

Documentation updates:

• Updated internal link rules in docs/04-adding-blog-post.md to instruct authors to write locale-neutral internal links and describe the new automatic locale prefixing behavior.

Supporting constants and patterns:

• Added constants for default locale, supported locale prefixes, and asset path detection in src/lib/markdown.ts to support the new link rewriting logic.

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