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>

alembic/versions/s7a8b9c0d1e2_add_storefront_locale_to_vendors.py

@@ -0,0 +1,40 @@
+# alembic/versions/s7a8b9c0d1e2_add_storefront_locale_to_vendors.py
+"""Add storefront_locale to vendors for currency formatting.
+
+Revision ID: s7a8b9c0d1e2
+Revises: r6f7a8b9c0d1
+Create Date: 2026-01-02 20:00:00.000000
+
+This migration adds a nullable storefront_locale field to vendors.
+NULL means the vendor inherits from platform defaults.
+Examples: 'fr-LU', 'de-DE', 'en-GB'
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = "s7a8b9c0d1e2"
+down_revision = "r6f7a8b9c0d1"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    """Add storefront_locale column to vendors table."""
+    # Nullable - NULL means "inherit from platform default"
+    op.add_column(
+        "vendors",
+        sa.Column(
+            "storefront_locale",
+            sa.String(10),
+            nullable=True,
+            comment="Currency/number formatting locale (NULL = inherit from platform)",
+        ),
+    )
+
+
+def downgrade() -> None:
+    """Remove storefront_locale column from vendors table."""
+    op.drop_column("vendors", "storefront_locale")