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-shared/src/application/applicationType.ts

@@ -21,6 +21,11 @@ export type ApplicationManifest = SyncableEntityOptions & {
   issueReportUrl?: string;
   postInstallLogicFunction?: PostInstallLogicFunctionApplicationManifest;
   preInstallLogicFunction?: PreInstallLogicFunctionApplicationManifest;
+  /**
+   * @deprecated Custom settings tabs are no longer supported. This property is
+   * kept for backward compatibility with older manifests but is now ignored.
+   * Use typed `applicationVariables` / `serverVariables` instead.
+   */
   settingsCustomTabFrontComponentUniversalIdentifier?: string;
   packageJsonChecksum: string | null;
   yarnLockChecksum: string | null;