fix(metadata): remove deprecated isCustom from Objects and Fields (#21799)

## Context

Follow-up to #21228, which deprecated `isCustom` on object/field
metadata but kept it exposed because the frontend still relied on it.
This removes it from the GraphQL API and the frontend entirely.

## Implementation

### Server
- Remove `isCustom` `@Field` from the `Object`, `Field`, and
`MinimalObjectMetadata` GraphQL types
- Remove the `isCustom` `@ResolveField` resolvers and the
`isCustomLoader` dataloader (+ payload/interface)
- Remove `isCustom` as an internal `@HideField()` on the Object/Field
DTOs used by the i18n standard-override gate > Use an explicit
isStandard instead (which is the correct gating)

### Frontend
- Add `getIsMetadataItemCustom` helper + `useGetIsMetadataItemCustom`
hook: an item is custom when `applicationId ===
currentWorkspace.workspaceCustomApplication.id`
- Migrate all consumers off `objectMetadataItem.isCustom` /
`fieldMetadataItem.isCustom`; `isRecordFieldReadOnly` now takes a
precomputed `isFieldCustom`
- Drop `isCustom` from the metadata fragment/mutations/minimal query, FE
types, zod schemas, and mock generators; regenerate GraphQL types

## Notes
- Breaking change on the (already-deprecated) `Object.isCustom` /
`Field.isCustom` GraphQL fields and the `isCustom` filter
- FE semantic is "belongs to the workspace custom app" (third-party-app
objects/fields are treated as non-custom)
- `isCustom` on IndexMetadata / View / Skill / Agent is a separate
column and is untouched
- Breaking changes on REST metadata API

packages/twenty-server/src/engine/metadata-modules/flat-object-metadata/utils/from-flat-object-metadata-to-object-metadata-dto.util.ts

@@ -1,6 +1,5 @@
 import { type FlatObjectMetadata } from 'src/engine/metadata-modules/flat-object-metadata/types/flat-object-metadata.type';
 import { type ObjectMetadataDTO } from 'src/engine/metadata-modules/object-metadata/dtos/object-metadata.dto';
-import { belongsToTwentyStandardApp } from 'src/engine/metadata-modules/utils/belongs-to-twenty-standard-app.util';
 
 export const fromFlatObjectMetadataToObjectMetadataDto = (
   flatObjectMetadata: FlatObjectMetadata,
@@ -37,7 +36,6 @@ export const fromFlatObjectMetadataToObjectMetadataDto = (
     id,
     universalIdentifier,
     isActive,
-    isCustom: !belongsToTwentyStandardApp(flatObjectMetadata),
     isLabelSyncedWithName,
     isRemote,
     isSearchable,