fix: prevent default object re-selection in relation field form (#17313)

Fixes #17111

### Problem
When creating a Relation field, after deselecting the pre-selected
default object (e.g., Company) and selecting a different object (e.g.,
Opportunity), both objects would end up being selected, showing "2
Objects" instead of just the newly selected one.

### Root Cause
The `initialMorphRelationsObjectMetadataIds` array was being recreated
on every component render, causing react-hook-form's `Controller` to
treat the `defaultValue` prop as a new value and re-apply it after user
changes.

### Solution
1. **Memoized the initial value**: Used `useMemo` to ensure
`initialMorphRelationsObjectMetadataIds` has a stable reference across
renders
2. **Added initialization guard**: Used `useEffect` with a `useRef` flag
to ensure the default value is only set once during component mount
3. **Maintained Controller defaultValue**: Kept the `defaultValue` prop
on the Controller, which now works correctly with the stable memoized
value

### Changes
- `SettingsDataModelFieldRelationForm.tsx`: Added memoization and
initialization guard to prevent default value re-application

---------

Co-authored-by: Etienne <45695613+etiennejouan@users.noreply.github.com>

packages/twenty-front/src/modules/settings/components/SettingsMorphRelationMultiSelect.tsx

@@ -1,5 +1,5 @@
 import styled from '@emotion/styled';
-import { plural } from '@lingui/core/macro';
+import { plural, t } from '@lingui/core/macro';
 import { useMemo, useRef, useState, type MouseEvent } from 'react';
 
 import { Dropdown } from '@/ui/layout/dropdown/components/Dropdown';
@@ -191,6 +191,7 @@ export const SettingsMorphRelationMultiSelect = ({
           isDisabled={isDisabled}
           selectSizeVariant={selectSizeVariant}
           hasRightElement={hasRightElement}
+          placeholderText={t`Select objects...`}
         />
       ) : (
         <Dropdown
@@ -213,6 +214,7 @@ export const SettingsMorphRelationMultiSelect = ({
               isDisabled={isDisabled}
               selectSizeVariant={selectSizeVariant}
               hasRightElement={hasRightElement}
+              placeholderText={t`Select objects...`}
             />
           }
           dropdownComponents={