feat: Add reusable tag cloud partial with A11y and dark/light mode support
- Create tag-cloud.html partial with flexible dict interface:
* showCount (bool): Toggle count badges
* wrapInWidget (bool): Sidebar widget wrapper with .sidebar-widget class
* maxTags (int): Limit shown tags (used for sidebar: 15 max)
* headingLevel (h2|h3): Configurable heading element
- Implement visual tier scaling by frequency (3 tiers):
* low: 0.75rem, 0.75 opacity — uncommon tags
* medium: 0.875rem, 0.88 opacity — moderate frequency
* high: 1rem, 1 opacity, accent border — popular tags
- Add .tag-cloud and .tag-tier-* CSS classes (main.css):
* Uses CSS variables (--accent, --border, --text-dim) for dark/light compatibility
* Focus ring matches site standard (outline-offset: 2px)
* Hover state: accent border + subtle bg tint
* prefers-reduced-motion: transitions disabled
- Integrate in 3 locations:
* Homepage (layouts/index.html): Full cloud with counts
* Article sidebar (layouts/partials/sidebar.html): Compact widget, 15 max, no counts
* 404 pages (404.en.html, 404.it.html): Full cloud between recent articles and nav
- A11y implementation:
* <section aria-labelledby> landmark (non-sidebar mode)
* <nav aria-label="Browse by topic"> named navigation
* Each link aria-label includes count text even when visual badge hidden
* <span aria-hidden="true"> on count badge to avoid duplication
* Proper heading hierarchy (h2 homepage, h3 on 404)
- Add i18n keys (en.yaml, it.yaml):
* tagCloud: "Explore Topics" / "Esplora gli argomenti"
* exploreTopics: "Browse by topic" / "Sfoglia per argomento"
- URL handling: Use .Page.RelPermalink from OrderedTaxonomyEntry — no manual /tags/ construction, language-aware paths work automatically
