feat: add configurable currency locale and fix vendor JS init

Currency Locale Configuration:
- Add platform-level storefront settings (locale, currency)
- Create PlatformSettingsService with resolution chain:
  vendor → AdminSetting → environment → hardcoded fallback
- Add storefront_locale nullable field to Vendor model
- Update shop routes to resolve and pass locale to templates
- Add window.SHOP_CONFIG for frontend JavaScript access
- Centralize formatPrice() in shop-layout.js using SHOP_CONFIG
- Remove local formatPrice functions from shop templates

Vendor JS Bug Fix:
- Fix vendorCode being null on all vendor pages
- Root cause: page components overriding init() without calling parent
- Add parent init call to 14 vendor JS files
- Add JS-013 architecture rule to prevent future regressions
- Validator now checks vendor JS files for parent init pattern

Files changed:
- New: app/services/platform_settings_service.py
- New: alembic/versions/s7a8b9c0d1e2_add_storefront_locale_to_vendors.py
- Modified: 14 vendor JS files, shop templates, validation scripts

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

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

models/database/vendor.py

@@ -121,6 +121,10 @@ class Vendor(Base, TimestampMixin):
         JSON, nullable=False, default=["fr", "de", "en"]
     )  # Array of enabled languages for storefront language selector
 
+    # Currency/number formatting locale (e.g., 'fr-LU' = "29,99 €", 'en-GB' = "€29.99")
+    # NULL means inherit from platform default (AdminSetting 'default_storefront_locale')
+    storefront_locale = Column(String(10), nullable=True)
+
     # ========================================================================
     # Relationships
     # ========================================================================