Refactor WiFiManager: extract server and handlers into separate modules

- Move server lifecycle management to WiFiManagerServer
- Move HTTP handlers and rendering to WiFiManagerHandlers
- Move DEBUG_WM template definitions to WiFiManagerDebug
- Implement lazy initialization of server manager to save memory
- Add proper cleanup in destructor and on multiple createServer calls
- Update tests to use getServer() and getDNSServer() accessors

test/test_wifimanager/tests/test_parameter_values.cpp

@@ -1,6 +1,7 @@
 #include <unity.h>
 #include <Arduino.h>
 #include <WiFiManager.h>
+#include <WiFiManagerParameter.h>
 #include "../test_main.h"
 
 // Test parameter value set directly