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/dashboard.js

@@ -38,13 +38,17 @@ function vendorDashboard() {
             }
             window._vendorDashboardInitialized = true;
 
-            // IMPORTANT: Call parent init first to set vendorCode from URL
-            const parentInit = data().init;
-            if (parentInit) {
-                await parentInit.call(this);
-            }
+            try {
+                // IMPORTANT: Call parent init first to set vendorCode from URL
+                const parentInit = data().init;
+                if (parentInit) {
+                    await parentInit.call(this);
+                }
 
-            await this.loadDashboardData();
+                await this.loadDashboardData();
+            } catch (error) {
+                vendorDashLog.error('Failed to initialize dashboard:', error);
+            }
         },
 
         async loadDashboardData() {
@@ -94,7 +98,11 @@ function vendorDashboard() {
         },
 
         async refresh() {
-            await this.loadDashboardData();
+            try {
+                await this.loadDashboardData();
+            } catch (error) {
+                vendorDashLog.error('Failed to refresh dashboard:', error);
+            }
         },
 
         formatCurrency(amount) {