refactor: centralize locale resolution through PlatformSettingsService

Remove hardcoded locale fallbacks from templates and ensure all routes
use the proper resolution chain (AdminSetting → Config → Default).

Changes:
- Add get_vendor_context() helper to vendor_pages.py mirroring shop pattern
- Update all vendor routes to use get_vendor_context() with db session
- Update shop routes to pass db session for locale resolution
- Remove hardcoded fallbacks from shop/base.html and vendor/base.html
- Templates now receive pre-resolved locale values from routes

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

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

app/templates/shop/base.html

@@ -308,12 +308,12 @@
     {# 1. Log Configuration (must load first) #}
     <script src="{{ url_for('static', path='shared/js/log-config.js') }}"></script>
 
-    {# 2. Global Shop Configuration (currency/locale settings) #}
+    {# 2. Global Shop Configuration (resolved via PlatformSettingsService) #}
     <script>
         window.SHOP_CONFIG = {
-            locale: '{{ storefront_locale | default("fr-LU") }}',
-            currency: '{{ storefront_currency | default("EUR") }}',
-            language: '{{ request.state.language|default("fr") }}'
+            locale: '{{ storefront_locale }}',
+            currency: '{{ storefront_currency }}',
+            language: '{{ request.state.language }}'
         };
     </script>
 

app/templates/vendor/base.html

@@ -50,6 +50,15 @@
     <!-- 1. FIRST: Log Configuration -->
     <script src="{{ url_for('static', path='shared/js/log-config.js') }}"></script>
 
+    <!-- 1.5: Vendor Configuration (resolved via PlatformSettingsService) -->
+    <script>
+        window.VENDOR_CONFIG = {
+            locale: '{{ storefront_locale }}',
+            currency: '{{ storefront_currency }}',
+            dashboardLanguage: '{{ dashboard_language }}'
+        };
+    </script>
+
     <!-- 2. SECOND: Icons (before Alpine.js) -->
     <script src="{{ url_for('static', path='shared/js/icons.js') }}"></script>