fix: context-aware back button for cross-module admin navigation

The tenancy merchant detail page now reads an optional ?back= query
parameter to determine the back button destination. Falls back to
/admin/merchants when no param is present (default behavior preserved).

The loyalty merchant detail "View Merchant" link now passes
?back=/admin/loyalty/merchants/{id} so clicking back from the tenancy
page returns to the loyalty context instead of the merchants list.

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

app/modules/tenancy/templates/tenancy/admin/merchant-detail.html

@@ -11,7 +11,7 @@
 {% block alpine_data %}adminMerchantDetail(){% endblock %}
 
 {% block content %}
-{% call detail_page_header("merchant?.name || 'Merchant Details'", '/admin/merchants', subtitle_show='merchant') %}
+{% call detail_page_header("merchant?.name || 'Merchant Details'", request.query_params.get('back', '/admin/merchants'), subtitle_show='merchant') %}
     ID: <span x-text="merchantId"></span>
     <span class="text-gray-400 mx-2">|</span>
     <span x-text="merchant?.store_count || 0"></span> store(s)