Add system view fallback (#18536)

## Context
The goal is to add a "See records" button in all objects that would
redirect to that view (this will be done in a later PR). See screenshot
below.
<img width="665" height="312" alt="Screenshot 2026-03-10 at 15 54 36"
src="https://github.com/user-attachments/assets/6e23a75b-cff0-4d93-bce8-b5481b05c6f6"
/>

## Implementation
- If a view does not exist on an object, there is a **temporary**
fallback where the frontend creates the missing view as a custom view
when going over the object index page
- System objects are now surfaced but we don't want their records to be
editable, they will be readonly (mostly, all fields will be non-editable
except for their custom fields).
- We can't create a new record of a system object, some actions are also
hidden.
- The backend now rejects if you are trying to delete the last view of
an object

packages/twenty-server/src/engine/workspace-manager/workspace-migration/workspace-migration-builder/validators/services/flat-view-validator.service.ts

@@ -149,6 +149,7 @@ export class FlatViewValidatorService {
     flatEntityToValidate,
     optimisticFlatEntityMapsAndRelatedFlatEntityMaps: {
       flatViewMaps: optimisticFlatViewMaps,
+      flatObjectMetadataMaps: optimisticFlatObjectMetadataMaps,
     },
   }: UniversalFlatEntityValidationArgs<
     typeof ALL_METADATA_NAME.view
@@ -172,6 +173,34 @@ export class FlatViewValidatorService {
         message: t`View not found`,
         userFriendlyMessage: msg`View not found`,
       });
+
+      return validationResult;
+    }
+
+    const parentObjectStillExists = isDefined(
+      findFlatEntityByUniversalIdentifier({
+        universalIdentifier: existingFlatView.objectMetadataUniversalIdentifier,
+        flatEntityMaps: optimisticFlatObjectMetadataMaps,
+      }),
+    );
+
+    if (parentObjectStillExists) {
+      const viewsForSameObject = Object.values(
+        optimisticFlatViewMaps.byUniversalIdentifier,
+      ).filter(
+        (view) =>
+          isDefined(view) &&
+          view.objectMetadataUniversalIdentifier ===
+            existingFlatView.objectMetadataUniversalIdentifier,
+      );
+
+      if (viewsForSameObject.length <= 1) {
+        validationResult.errors.push({
+          code: ViewExceptionCode.INVALID_VIEW_DATA,
+          message: t`Cannot delete the only view for this object`,
+          userFriendlyMessage: msg`Cannot delete the only view for this object`,
+        });
+      }
     }
 
     return validationResult;