feat(applications): remove the application custom settings tab (#22156)

## Summary

Removes the application **custom settings tab** feature. This is one
half of #22059, split out so it can be reviewed/merged independently
from the variable-types enrichment.

## Changes

- Remove the `SettingsApplicationCustomTab` component and its tab
entry/rendering in `SettingsApplicationDetails`.
- Stop syncing `settingsCustomTabFrontComponent` from application
manifests — `ApplicationManifestMigrationService` now only syncs the
default role.
- Deprecate the now-unused fields (kept for backward compatibility, no
longer read or synced):
- `ApplicationDTO.settingsCustomTabFrontComponentId` (GraphQL
`@deprecated`)
-
`ApplicationManifest.settingsCustomTabFrontComponentUniversalIdentifier`
- the `settingsCustomTabFrontComponentId` column comment on
`ApplicationEntity`

The DB column is intentionally **not dropped**, so existing
installations upgrade cleanly.

packages/twenty-front/src/generated-metadata/graphql.ts

@@ -302,6 +302,7 @@ export type Application = {
   objects: Array<Object>;
   packageJsonChecksum?: Maybe<Scalars['String']['output']>;
   packageJsonFileId?: Maybe<Scalars['UUID']['output']>;
+  /** @deprecated Custom settings tabs are no longer supported. This field is ignored. */
   settingsCustomTabFrontComponentId?: Maybe<Scalars['UUID']['output']>;
   universalIdentifier: Scalars['String']['output'];
   version?: Maybe<Scalars['String']['output']>;