## 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
Weiko