fix(hosting): site detail null guard + cleaner preview URLs

- Site detail template: x-show → x-if to prevent Alpine evaluating
  expressions when site is null during async loading
- Slugify: prefer domain_name over business_name for subdomain
  generation (batirenovation-strasbourg vs bati-rnovation-strasbourg-
  peinture-ravalement-dans). Cap at 30 chars. Strip protocol/TLD.
- POC builder passes domain_name for clean slugs
- Remove .lu/.fr/.com TLD from slugs automatically

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/modules/hosting/services/poc_builder_service.py

@@ -60,6 +60,7 @@ class PocBuilderService:
         # 4. Create HostedSite + Store
         site_data = {
             "business_name": context["business_name"],
+            "domain_name": prospect.domain_name,  # used for clean subdomain slug
             "prospect_id": prospect_id,
             "contact_email": context.get("email"),
             "contact_phone": context.get("phone"),