fix: preserve default sync folder path when no overriding

The spread operator `...` replaces `baseConfig.webDav` with
`defaultOverride.webDav` entirely, instead of merging them. Hence, the
default sync folder path in `baseConfig.webDav` is wiped out.

This patch fixes this issue by manually providing a merged `webDav`.

Fix: The second issue in https://github.com/johannesjo/super-productivity/issues/4545#issuecomment-2974843258

src/app/imex/sync/sync-config.service.ts

@@ -42,6 +42,10 @@ export class SyncConfigService {
               return {
                 ...baseConfig,
                 ...defaultOverride,
+                webDav: {
+                  ...baseConfig.webDav,
+                  ...defaultOverride.webDav,
+                },
                 encryptKey: '',
               };
             })