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-client-sdk/src/metadata/generated/schema.graphql

@@ -714,7 +714,7 @@ type Application {
   applicationRegistrationId: UUID
   canBeUninstalled: Boolean!
   defaultRoleId: String
-  settingsCustomTabFrontComponentId: UUID
+  settingsCustomTabFrontComponentId: UUID @deprecated(reason: "Custom settings tabs are no longer supported. This field is ignored.")
   defaultLogicFunctionRole: Role
   agents: [Agent!]!
   frontComponents: [FrontComponent!]!

packages/twenty-client-sdk/src/metadata/generated/schema.ts

@@ -475,6 +475,7 @@ export interface Application {
     applicationRegistrationId?: Scalars['UUID']
     canBeUninstalled: Scalars['Boolean']
     defaultRoleId?: Scalars['String']
+    /** @deprecated Custom settings tabs are no longer supported. This field is ignored. */
     settingsCustomTabFrontComponentId?: Scalars['UUID']
     defaultLogicFunctionRole?: Role
     agents: Agent[]
@@ -3510,6 +3511,7 @@ export interface ApplicationGenqlSelection{
     applicationRegistrationId?: boolean | number
     canBeUninstalled?: boolean | number
     defaultRoleId?: boolean | number
+    /** @deprecated Custom settings tabs are no longer supported. This field is ignored. */
     settingsCustomTabFrontComponentId?: boolean | number
     defaultLogicFunctionRole?: RoleGenqlSelection
     agents?: AgentGenqlSelection