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>

website/templates/index.html

@@ -113,6 +113,7 @@
 </section>
 {% endif %}
 
+<script type="application/json" id="filter-urls">{{ filter_urls_json | safe }}</script>
 <section class="results-section" id="library-index">
   <div class="results-intro section-shell" data-reveal>
     <div>