fix: restore articles to taxonomy pages and fix i18n headings
Fixed two critical bugs in taxonomy layouts:
1. Pinned/unpinned filter was silently hiding all articles. The filter
only matched pages that explicitly set pinned=false, dropping all
articles that omitted the parameter (most articles have nil, not false).
Added the nil/empty fallback from _default/list.html.
2. Missing i18n keys caused blank headings on some taxonomy pages:
- Added 'tag' key for /tags/term/ pages
- Added 'categories' key for /categories/ list page
3. Harmonized spacing with main article list for visual consistency:
Changed from space-y-2 max-w-2xl to space-y-6 max-w-3xl
