fix: improve error handling and display in desktop client enrollment flow

Add formatInvokeError helper function to handle various error types from Tauri invoke calls with fallback messages. Update enroll_device to include response body in error message when enrollment fails with non-success status. Add windows_subsystem attribute to main.rs to suppress console window in release builds on Windows.

desktop-client/src-tauri/src/lib.rs

@@ -153,7 +153,12 @@ async fn enroll_device(
         .map_err(|err| format!("Enrollment failed: {}", err))?;
 
     if !enroll_response.status().is_success() {
-        return Err(format!("Enrollment failed with status {}", enroll_response.status()));
+        let status = enroll_response.status();
+        let body = enroll_response
+            .text()
+            .await
+            .unwrap_or_else(|_| "<unable to read response body>".into());
+        return Err(format!("Enrollment failed with status {}: {}", status, body));
     }
 
     let enroll = enroll_response

desktop-client/src-tauri/src/main.rs

@@ -1,3 +1,5 @@
+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")]
+
 fn main() {
     nexavpn_desktop::run();
 }