Solves [Sonarly Issue 8116](https://sonarly.com/issue/8116).
### Problem
Editing a morph relation field (e.g. "Parent Object" on Task) via the
field widget was broken in two ways:
1. **Setting a value** sent the wrong foreign key name (`parentObjectId`
instead of target-specific keys like `parentObjectCompanyId`), causing
the relation to not save.
2. **Detaching** never sent a request at all — the early return check
`valueToPersist?.id === currentValue?.id` evaluated to `undefined ===
undefined` when the morph field wasn't loaded in the store, silently
skipping the update.
The record detail section worked fine because it uses a separate hook
(`useMorphPersistManyToOne`).
### Fix
Added proper morph relation handling in `usePersistField` so all
persistence goes through this single hook consistently:
- Compute the correct FK name using `computeMorphRelationFieldName`
(e.g. `parentObjectCompanyId`) instead of deriving it from the field
name directly.
- Null all morph FK columns before setting the target one, ensuring only
one FK is non-null at a time (consistent with
`useMorphPersistManyToOne`).
- Fix the early return to only skip when **setting** a value that
matches the current one — detach always proceeds.
- Derive `currentRelationId` via a type guard instead of an `as` cast.
