fix: nettoyer warning state_clone + xcopy /Y + taskkill avant copie runtime

- main.rs: supprimer state_clone unused dans restart_backend
- build-windows.bat: taskkill /F python.exe avant xcopy (évite Access denied)
- build-windows.bat: /Y sur xcopy (pas de prompt interactif)
- build-windows.bat: 2>nul sur rmdir (silencieux si dossier absent)

desktop/src/main.rs

@@ -256,9 +256,7 @@ fn restart_backend(state: tauri::State<BackendProcess>) -> Result<String, String
             *guard = Some(new_child);
             drop(guard);
 
-            // Wait for it to become ready (fire and forget with UI poll)
-            let state_clone = state.inner().clone();
-            // Can't clone state directly, use a flag pattern
+            // Fire and forget — the monitoring loop will detect when it's ready
             info!("Backend restart initiated");
             Ok("restarting".to_string())
         }