Add super-admin Administration section and restructure global vs company settings

- Add Administration sidebar section (super-admin only) with Companies, Users, and Global Settings pages
- Add super-admin middleware, controllers, and API routes under /api/v1/super-admin/
- Allow super-admins to manage all companies and users across tenants
- Add user impersonation with short-lived tokens, audit logging, and UI banner
- Move system-level settings (Mail, PDF, Backup, Update, File Disk) from per-company to Administration > Global Settings
- Convert save_pdf_to_disk from CompanySetting to global Setting
- Add per-company mail configuration overrides (optional, falls back to global)
- Add CompanyMailConfigService to apply company mail config before sending emails

app/Http/Middleware/SuperAdminMiddleware.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace App\Http\Middleware;
+
+use Auth;
+use Closure;
+use Illuminate\Http\Request;
+use Symfony\Component\HttpFoundation\Response;
+
+class SuperAdminMiddleware
+{
+    public function handle(Request $request, Closure $next, $guard = null): Response
+    {
+        if (Auth::guard($guard)->guest() || ! Auth::user()->isSuperAdmin()) {
+            return response()->json(['error' => 'unauthorized'], 403);
+        }
+
+        return $next($request);
+    }
+}