fix: use single quotes for x-data attributes with tojson

The Jinja |tojson filter outputs JSON with double quotes. When used
inside a double-quoted HTML attribute, these quotes break the attribute
parsing causing "expected expression, got '}'" errors.

Solution: Use single quotes for x-data attributes so JSON double quotes
don't conflict:
  <div x-data='languageSelector("fr", {{ langs|tojson }})'>

Updated:
- language_selector.html macro (all 3 variants)
- shop/base.html language selector
- LANG-002 and LANG-003 architecture rules documentation
- Validator to detect double-quoted x-data with tojson

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/templates/shop/base.html

@@ -136,7 +136,7 @@
                     {# Language Selector #}
                     {% set enabled_langs = vendor.storefront_languages if vendor and vendor.storefront_languages else ['fr', 'de', 'en'] %}
                     {% if enabled_langs|length > 1 %}
-                    <div class="relative" x-data="languageSelector('{{ request.state.language|default('fr') }}', {{ enabled_langs|tojson }})">
+                    <div class="relative" x-data='languageSelector("{{ request.state.language|default("fr") }}", {{ enabled_langs|tojson }})'>
                         <button
                             @click="isLangOpen = !isLangOpen"
                             @click.outside="isLangOpen = false"