refactor(website): extract render_category, replace slugify filter with filter_urls map

- Extract render_category() helper to deduplicate the three category/group/builtin
  rendering blocks in build.py
- Replace synthetic dict literals with synthetic_category() helper
- Rewrite subcategory rendering to avoid O(n²) loop using precomputed dicts
- Pass filter_urls (not just JSON) to templates so Jinja can look up group URLs
  directly instead of applying the slugify filter at render time
- Remove slugify from env.filters (no longer used in templates)
- Replace isIndexPage() wrapper with isIndexDocument constant in main.js
- Fix: call applyFilters() on page load when activeFilter is set
- Remove dead else branch in tag click handler (category pages with no URL)
- Switch .hero-category-links from CSS columns to CSS grid for more even layout
- Remove max-width cap on .category-subtitle

Co-Authored-By: Claude <noreply@anthropic.com>