fix: resolve all JS architecture violations (JS-005 through JS-009)

Fixed 89 violations across vendor, admin, and shared JavaScript files:

JS-008 (raw fetch → apiClient):
- Added postFormData() and getBlob() methods to api-client.js
- Updated inventory.js, messages.js to use apiClient.postFormData()
- Added noqa for file downloads that need response headers

JS-009 (window.showToast → Utils.showToast):
- Updated admin/messages.js, notifications.js, vendor/messages.js
- Replaced alert() in customers.js

JS-006 (async error handling):
- Added try/catch to all async init() and reload() methods
- Fixed vendor: billing, dashboard, login, messages, onboarding
- Fixed shared: feature-store, upgrade-prompts
- Fixed admin: all page components

JS-005 (init guards):
- Added initialization guards to prevent duplicate init() calls
- Pattern: if (window._componentInitialized) return;

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

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

static/vendor/js/billing.js

@@ -29,22 +29,30 @@ function vendorBilling() {
 
         // Initialize
         async init() {
-            // Check URL params for success/cancel
-            const params = new URLSearchParams(window.location.search);
-            if (params.get('success') === 'true') {
-                this.showSuccessMessage = true;
-                window.history.replaceState({}, document.title, window.location.pathname);
-            }
-            if (params.get('cancelled') === 'true') {
-                this.showCancelMessage = true;
-                window.history.replaceState({}, document.title, window.location.pathname);
-            }
-            if (params.get('addon_success') === 'true') {
-                this.showAddonSuccessMessage = true;
-                window.history.replaceState({}, document.title, window.location.pathname);
-            }
+            // Guard against multiple initialization
+            if (window._vendorBillingInitialized) return;
+            window._vendorBillingInitialized = true;
 
-            await this.loadData();
+            try {
+                // Check URL params for success/cancel
+                const params = new URLSearchParams(window.location.search);
+                if (params.get('success') === 'true') {
+                    this.showSuccessMessage = true;
+                    window.history.replaceState({}, document.title, window.location.pathname);
+                }
+                if (params.get('cancelled') === 'true') {
+                    this.showCancelMessage = true;
+                    window.history.replaceState({}, document.title, window.location.pathname);
+                }
+                if (params.get('addon_success') === 'true') {
+                    this.showAddonSuccessMessage = true;
+                    window.history.replaceState({}, document.title, window.location.pathname);
+                }
+
+                await this.loadData();
+            } catch (error) {
+                billingLog.error('Failed to initialize billing page:', error);
+            }
         },
 
         async loadData() {