fix: double free WebDAVHandler (#1093)

## Summary

Ref:
https://github.com/crosspoint-reader/crosspoint-reader/pull/1047#discussion_r2838439305

To reproduce:
1. Open file transfer
2. Join a network
3. Once it's connected, press (hold) back

---

### AI Usage

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please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? **NO**

src/network/CrossPointWebServer.cpp

@@ -12,6 +12,7 @@
 
 #include "CrossPointSettings.h"
 #include "SettingsList.h"
+#include "WebDAVHandler.h"
 #include "html/FilesPageHtml.generated.h"
 #include "html/HomePageHtml.generated.h"
 #include "html/SettingsPageHtml.generated.h"
@@ -162,7 +163,7 @@ void CrossPointWebServer::begin() {
   // Collect WebDAV headers and register handler
   const char* davHeaders[] = {"Depth", "Destination", "Overwrite", "If", "Lock-Token", "Timeout"};
   server->collectHeaders(davHeaders, 6);
-  server->addHandler(&davHandler);
+  server->addHandler(new WebDAVHandler());  // Note: WebDAVHandler will be deleted by WebServer when server is stopped
   LOG_DBG("WEB", "WebDAV handler initialized");
 
   server->begin();