feat: refactor super admin to use is_super_admin flag and enhance site deletion logic

- Changed super admin detection from role-based to `is_super_admin` flag in AdminUser model
- Added `requireDefaultAdminSiteId()` method to throw validation error when no integration site exists
- Enhanced site deletion to migrate platform role bindings to fallback site and auto-delete site-specific admin accounts
- Made agent line code optional with auto-generation fallback using `{site_code}-agent-{counter}` format

tests/Unit/SiteAdminDefaultRolePermissionsTest.php

@@ -0,0 +1,13 @@
+<?php
+
+use App\Support\SiteAdminDefaultRolePermissions;
+
+test('site admin template includes dashboard and settlement manage', function (): void {
+    $slugs = SiteAdminDefaultRolePermissions::templateSlugs();
+
+    expect($slugs)
+        ->toContain('prd.dashboard.view')
+        ->toContain('prd.agent.manage')
+        ->toContain('prd.settlement.agent.manage')
+        ->toContain('prd.report.view');
+});