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>

.architecture-rules/frontend.yaml

@@ -77,6 +77,60 @@ javascript_rules:
       file_pattern: "static/**/js/**/*.js"
       check: "async_error_handling"
 
+  - id: "JS-013"
+    name: "Components overriding init() must call parent init"
+    severity: "error"
+    description: |
+      When an Alpine.js component spreads ...data() and defines its own init() method,
+      it MUST call the parent init() first. The parent init() sets critical properties
+      like vendorCode (from URL), currentUser, and theme preference.
+
+      Without calling parent init(), properties like vendorCode will be null, causing
+      API calls like `/vendor/${this.vendorCode}/settings` to fail with
+      "Endpoint not found: /api/v1/vendor/null/settings".
+
+      WRONG (parent init never called):
+        function vendorSettings() {
+            return {
+                ...data(),
+                async init() {
+                    await this.loadSettings(); // this.vendorCode is null!
+                }
+            };
+        }
+
+      RIGHT (call parent init first):
+        function vendorSettings() {
+            return {
+                ...data(),
+                async init() {
+                    // IMPORTANT: Call parent init first to set vendorCode from URL
+                    const parentInit = data().init;
+                    if (parentInit) {
+                        await parentInit.call(this);
+                    }
+
+                    await this.loadSettings(); // this.vendorCode is now set
+                }
+            };
+        }
+
+      This pattern is required for ALL page-specific JavaScript files that:
+      1. Use ...data() to inherit base layout functionality
+      2. Define their own init() method
+    pattern:
+      file_pattern: "static/vendor/js/**/*.js"
+      check: "parent_init_call"
+      required_when:
+        - "contains: '...data()'"
+        - "contains: 'async init()'"
+      required_pattern:
+        - "data\\(\\)\\.init"
+        - "parentInit"
+      exceptions:
+        - "init-alpine.js"
+        - "login.js"
+
   - id: "JS-007"
     name: "Set loading state before async operations"
     severity: "warning"