feat: plugin controller config system with guest_comm_config hook integration

- Add HasPluginConfig trait and PluginController base class
- Integrate guest_comm_config hook in CommController for plugin frontend config injection
- Add user creation functionality to UserService and fix null value handling
- Enhance AbstractPlugin.getConfig() with key parameter support
- Multiple service layer optimizations and architecture improvements

app/Services/MailService.php

@@ -30,7 +30,7 @@ class MailService
     /**
      * 分块处理用户提醒邮件
      */
-    public function processUsersInChunks(int $chunkSize, callable $progressCallback = null): array
+    public function processUsersInChunks(int $chunkSize, ?callable $progressCallback = null): array
     {
         $statistics = [
             'processed_users' => 0,