feat: enhance NVS and WiFi handlers for improved credential management and error handling

main/network/wifi_handler.h

@@ -51,6 +51,8 @@ private:
   SemaphoreHandle_t credential_mutex = nullptr;
   // current connected / preferred SSID
   std::string current_ssid;
+  // current password (temporarily stored for successful connection event)
+  std::string current_password;
   // prevent auto-reconnect on expected disconnection, e.g. when user calls disconnect()
   // should be reset to false after connect()
   bool expect_disconnected = false;