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/templates/hosting/admin/site-detail.html

@@ -12,7 +12,8 @@
 {{ loading_state('Loading site...') }}
 {{ error_state('Error loading site') }}
 
-<div x-show="!loading && !error && site" class="space-y-6">
+<template x-if="!loading && !error && site">
+<div class="space-y-6">
     <!-- Header -->
     <div class="flex flex-col sm:flex-row sm:items-center justify-between my-6 gap-4">
         <div class="flex items-center space-x-4">
@@ -187,6 +188,7 @@
         </div>
     </div>
 </div>
+</template>
 
 <!-- Send Proposal Modal -->
 {% call modal('proposalModal', 'Send Proposal', show_var='showProposalModal', size='md', show_footer=false) %}