feat: add admin messaging interface

- Add admin messages API endpoints (/api/v1/admin/messages)
- Add admin messages page route (/admin/messages)
- Add messages.html template with split-panel conversation view
- Add messages.js Alpine component for messaging UI
- Add Messages link to admin sidebar under Platform Administration
- Add message icon with unread badge to admin header
- Update init-alpine.js with headerMessages component

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

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

static/admin/js/init-alpine.js

@@ -63,6 +63,7 @@ function data() {
     companies: 'platformAdmin',
     vendors: 'platformAdmin',
     users: 'platformAdmin',
+    messages: 'platformAdmin',
     // Vendor Operations (Products, Customers, Inventory, Orders, Shipping)
     'marketplace-products': 'vendorOps',
     'vendor-products': 'vendorOps',
@@ -221,4 +222,39 @@ function languageSelector(currentLang, enabledLanguages) {
 }
 
 // Export to window for use in templates
-window.languageSelector = languageSelector;
+window.languageSelector = languageSelector;
+
+/**
+ * Header messages badge component
+ * Shows unread message count in header
+ */
+function headerMessages() {
+    return {
+        unreadCount: 0,
+        pollInterval: null,
+
+        async init() {
+            await this.fetchUnreadCount();
+            // Poll every 30 seconds
+            this.pollInterval = setInterval(() => this.fetchUnreadCount(), 30000);
+        },
+
+        destroy() {
+            if (this.pollInterval) {
+                clearInterval(this.pollInterval);
+            }
+        },
+
+        async fetchUnreadCount() {
+            try {
+                const response = await apiClient.get('/admin/messages/unread-count');
+                this.unreadCount = response.total_unread || 0;
+            } catch (error) {
+                // Silently fail - don't spam console for auth issues on login page
+            }
+        }
+    };
+}
+
+// Export to window
+window.headerMessages = headerMessages;