feat(website): embed filter-to-url map in index for client routing

Adds filter_urls dict (categories, groups, subcategories) in build.py,
passes filter_urls_json to the template, and injects a JSON script block
before the results section in index.html. Covered by a new test that
verifies all three URL types are present and correctly resolved.

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