Add ENG / CAST / CAT static portfolio routes

[DidacLL]

Draft validation PR for the multilingual layer built on top of DidacLl/content-audit.

Scope:

• preserve English routes without a prefix;
• add /es/... and /ca/... using the same technical path segments and slugs;
• keep Markdown as the content owner, with locale-specific files under es/ and ca/;
• localize shared navigation/UI strings without introducing a client i18n runtime;
• add language switching, lang, canonical, hreflang and x-default metadata;
• require translation parity for the current public pages/projects/posts/CV views;
• make route discovery and Markdown-link validation multilingual-aware.

This PR temporarily targets main so the existing PR-only CI can validate the branch. Once checks are green it will be retargeted to DidacLl/content-audit so the multilingual diff remains isolated from the editorial rewrite.