fix: crash on startup due to uninitialized m_config pointer

The m_config member variable contained garbage value instead of
nullptr because C++ doesn't auto-initialize pointer members.
This caused invalid memory access and crash during startup.

Initialize m_config to nullptr to ensure proper initial state.

Author: Dami-star

src/seat/helper.h

@@ -302,7 +302,7 @@ private Q_SLOTS:
     void updateIdleInhibitor();
 
     static Helper *m_instance;
-    TreelandConfig *m_config;
+    TreelandConfig *m_config = nullptr;
 
     CurrentMode m_currentMode{ CurrentMode::Normal };