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/content-page-edit.js

@@ -37,9 +37,6 @@ function vendorContentPageEditor(pageId) {
 
         // Initialize
         async init() {
-            contentPageEditLog.info('=== VENDOR CONTENT PAGE EDITOR INITIALIZING ===');
-            contentPageEditLog.info('Page ID:', this.pageId);
-
             // Prevent multiple initializations
             if (window._vendorContentPageEditInitialized) {
                 contentPageEditLog.warn('Content page editor already initialized, skipping...');
@@ -47,17 +44,24 @@ function vendorContentPageEditor(pageId) {
             }
             window._vendorContentPageEditInitialized = true;
 
-            if (this.pageId) {
-                // Edit mode - load existing page
-                contentPageEditLog.group('Loading page for editing');
-                await this.loadPage();
-                contentPageEditLog.groupEnd();
-            } else {
-                // Create mode - use default values
-                contentPageEditLog.info('Create mode - using default form values');
-            }
+            try {
+                contentPageEditLog.info('=== VENDOR CONTENT PAGE EDITOR INITIALIZING ===');
+                contentPageEditLog.info('Page ID:', this.pageId);
 
-            contentPageEditLog.info('=== VENDOR CONTENT PAGE EDITOR INITIALIZATION COMPLETE ===');
+                if (this.pageId) {
+                    // Edit mode - load existing page
+                    contentPageEditLog.group('Loading page for editing');
+                    await this.loadPage();
+                    contentPageEditLog.groupEnd();
+                } else {
+                    // Create mode - use default values
+                    contentPageEditLog.info('Create mode - using default form values');
+                }
+
+                contentPageEditLog.info('=== VENDOR CONTENT PAGE EDITOR INITIALIZATION COMPLETE ===');
+            } catch (error) {
+                contentPageEditLog.error('Failed to initialize content page editor:', error);
+            }
         },
 
         // Load existing page