Reset Tab Page Layout (#19453)

## Context
- Add "Reset to default" for page layout tabs and widgets — backend
mutation resets overrides, reactivates deactivated children, and deletes
custom children
- Fix override write bug where mutating an overridable property (e.g.
widget title) on a standard-app entity incorrectly overwrote the base
column instead of writing to the overrides JSONB —
PageLayoutUpdateService now uses resolveFlatEntityOverridableProperties
for accurate diffing and routes properties through
sanitizeOverridableEntityInput
- Deprecate isOverridden - We need more time to think about this
feature. Currently this adds too much complexity for a very small
benefit



https://github.com/user-attachments/assets/a84546c8-1e15-4d9e-a489-0825cf8b8ed2

packages/twenty-server/src/engine/metadata-modules/utils/resolve-flat-entity-overridable-properties.util.ts

@@ -0,0 +1,21 @@
+import { isDefined } from 'twenty-shared/utils';
+
+type FlatEntityWithOverrides = {
+  [key: string]: unknown;
+  overrides: Record<string, unknown> | null;
+};
+
+export const resolveFlatEntityOverridableProperties = <
+  T extends FlatEntityWithOverrides,
+>(
+  flatEntity: T,
+): T => {
+  if (!isDefined(flatEntity.overrides)) {
+    return flatEntity;
+  }
+
+  return {
+    ...flatEntity,
+    ...flatEntity.overrides,
+  };
+};