feat: add platform selection frontend for platform admins

Frontend implementation of platform admin flow:
- Update login.js to check for platform selection after login
- Add platform selection page (/admin/select-platform)
- Add platform context indicator in admin header
- Add is_super_admin to UserResponse schema
- Show "Super Admin" badge or platform name with switch option

Platform admins now:
1. Login normally at /admin/login
2. Get redirected to /admin/select-platform if they have multiple platforms
3. See current platform in header with option to switch

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

app/templates/admin/partials/header.html

@@ -20,6 +20,27 @@
         </div>
 
         <ul class="flex items-center flex-shrink-0 space-x-6">
+            <!-- Platform Context Indicator (for platform admins) -->
+            <li class="flex" x-data="platformContext()">
+                <template x-if="platform && !isSuperAdmin">
+                    <div class="flex items-center space-x-2">
+                        <span class="px-3 py-1 text-xs font-medium text-purple-700 bg-purple-100 dark:text-purple-300 dark:bg-purple-900/30 rounded-full">
+                            <span x-text="platform.name"></span>
+                        </span>
+                        <a href="/admin/select-platform"
+                           class="text-xs text-gray-500 hover:text-purple-600 dark:text-gray-400 dark:hover:text-purple-400"
+                           title="Switch platform">
+                            <span x-html="$icon('switch-horizontal', 'w-4 h-4')"></span>
+                        </a>
+                    </div>
+                </template>
+                <template x-if="isSuperAdmin">
+                    <span class="px-3 py-1 text-xs font-medium text-green-700 bg-green-100 dark:text-green-300 dark:bg-green-900/30 rounded-full">
+                        Super Admin
+                    </span>
+                </template>
+            </li>
+
             <!-- Theme toggler -->
             <li class="flex">
                 <button class="rounded-md focus:outline-none focus:shadow-outline-purple"
@@ -182,6 +203,53 @@
 
 <!-- Header notifications and logout handler script -->
 <script>
+// Platform context component for header
+function platformContext() {
+    return {
+        platform: null,
+        isSuperAdmin: false,
+
+        init() {
+            // Try to get platform from localStorage
+            const storedPlatform = localStorage.getItem('admin_platform');
+            if (storedPlatform) {
+                try {
+                    this.platform = JSON.parse(storedPlatform);
+                } catch (e) {
+                    console.warn('Failed to parse stored platform:', e);
+                }
+            }
+
+            // Check if user is super admin
+            const storedUser = localStorage.getItem('admin_user');
+            if (storedUser) {
+                try {
+                    const user = JSON.parse(storedUser);
+                    this.isSuperAdmin = user.is_super_admin === true;
+                } catch (e) {
+                    console.warn('Failed to parse stored user:', e);
+                }
+            }
+        }
+    };
+}
+
+// Header messages component
+function headerMessages() {
+    return {
+        unreadCount: 0,
+
+        async init() {
+            try {
+                const response = await apiClient.get('/admin/messages/unread-count');
+                this.unreadCount = response.count || 0;
+            } catch (error) {
+                // Silently fail - messages count is not critical
+            }
+        }
+    };
+}
+
 // Header notifications component
 function headerNotifications() {
     return {
@@ -288,6 +356,7 @@ document.addEventListener('alpine:init', () => {
                     // Keep admin_last_visited_page so user returns to same page after login
                     localStorage.removeItem('admin_token');
                     localStorage.removeItem('admin_user');
+                    localStorage.removeItem('admin_platform');
                     // Note: Do NOT use localStorage.clear() - it would clear vendor/customer tokens too
                     window.location.href = '/admin/login';
                 });