feat: unify moba workspace and persist session tree layout

backend/src/main/java/com/sshmanager/service/ConnectionService.java

@@ -100,15 +100,17 @@ public class ConnectionService {
         return ConnectionDto.fromEntity(conn);
     }
 
-    @Transactional
-    public void delete(Long id, Long userId) {
-        Connection conn = connectionRepository.findById(id).orElseThrow(
-            () -> new RuntimeException("Connection not found: " + id));
-        if (!conn.getUserId().equals(userId)) {
-            throw new RuntimeException("Access denied");
-        }
-        connectionRepository.delete(conn);
-    }
+    @Transactional
+    public void delete(Long id, Long userId) {
+        Connection conn = connectionRepository.findById(id).orElse(null);
+        if (conn == null) {
+            return;
+        }
+        if (!conn.getUserId().equals(userId)) {
+            throw new RuntimeException("Access denied");
+        }
+        connectionRepository.delete(conn);
+    }
 
     public Connection getConnectionForSsh(Long id, Long userId) {
         Connection conn = connectionRepository.findById(id).orElseThrow(