fix inconsistent widget placeholder placement (#15580)

fixes -
https://github.com/twentyhq/twenty/pull/15496#issuecomment-3481636780

and https://discord.com/channels/1130383047699738754/1434971632539009055

- make the pending placeholder to be non-static -- so that it behaves
similarly to compact behavior like other widgets do
- make sure users can't set dashboards to be opened in the side panel --
similar to workflows

before- 


https://github.com/user-attachments/assets/d991db56-1388-4e4b-b743-a63f56b6187a

after- 


https://github.com/user-attachments/assets/9031f2c6-1e3b-4604-a5a0-3124d14484ce

packages/twenty-front/src/modules/object-record/utils/canOpenObjectInSidePanel.ts

@@ -1,5 +1,6 @@
 export const canOpenObjectInSidePanel = (objectNameSingular: string) =>
   !(
     objectNameSingular === 'workflow' ||
-    objectNameSingular === 'workflowVersion'
+    objectNameSingular === 'workflowVersion' ||
+    objectNameSingular === 'dashboard'
   );