fix(ui): provide default branding for central admin

app/Http/Middleware/HandleInertiaRequests.php

@@ -65,7 +65,12 @@ class HandleInertiaRequests extends Middleware
             'branding' => function () {
                 $tenant = tenancy()->tenant;
                 if (!$tenant) {
-                    return null;
+                    // 中央後台預設 Branding
+                    return [
+                        'logo_url' => \Storage::url('defaults/logo.png'), // 中央後台也使用預設 Logo
+                        'primary_color' => '#4F46E5',
+                        'text_color' => '#1a1a1a',
+                    ];
                 }
 
                 $logoUrl = null;