fix(registration): time-limit first root account creation

Open root registration for ten minutes after startup, then require a Coolify restart to reopen it. Update installer credential handling and explain the setup window in the registration UI.

database/seeders/RootUserSeeder.php

@@ -25,9 +25,12 @@ class RootUserSeeder extends Seeder
                 return;
             }
 
+            // The installer writes an empty ROOT_USERNAME line, so an empty value must also use the default.
+            $username = env('ROOT_USERNAME') ?: 'Root User';
+
             $validator = Validator::make([
                 'email' => env('ROOT_USER_EMAIL'),
-                'username' => env('ROOT_USERNAME', 'Root User'),
+                'username' => $username,
                 'password' => env('ROOT_USER_PASSWORD'),
             ], [
                 'email' => ['required', 'email:rfc,dns', 'max:255'],
@@ -48,7 +51,7 @@ class RootUserSeeder extends Seeder
             try {
                 $user = (new User)->forceFill([
                     'id' => 0,
-                    'name' => env('ROOT_USERNAME', 'Root User'),
+                    'name' => $username,
                     'email' => env('ROOT_USER_EMAIL'),
                     'password' => Hash::make(env('ROOT_USER_PASSWORD')),
                 ]);