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/App.tsx

@@ -11,6 +11,18 @@ type EnrollmentState = {
   tunnelStrategy: string;
 };
 
+function formatInvokeError(err: unknown, fallback: string) {
+  if (typeof err === "string" && err.trim().length > 0) {
+    return err;
+  }
+
+  if (err instanceof Error && err.message.trim().length > 0) {
+    return err.message;
+  }
+
+  return fallback;
+}
+
 export function App() {
   const [serverUrl, setServerUrl] = useState("http://localhost");
   const [username, setUsername] = useState("");
@@ -41,7 +53,7 @@ export function App() {
       });
       setState(result);
     } catch (err) {
-      setError(err instanceof Error ? err.message : "Enrollment failed");
+      setError(formatInvokeError(err, "Enrollment failed"));
     } finally {
       setLoading(false);
     }
@@ -54,7 +66,7 @@ export function App() {
       setConnected((value) => !value);
       setError(null);
     } catch (err) {
-      setError(err instanceof Error ? err.message : "Tunnel action failed");
+      setError(formatInvokeError(err, "Tunnel action failed"));
     }
   }