feat(sdk): add definePageLayoutTab for extending existing page layouts (#20004)

## Summary

Introduces `definePageLayoutTab` so apps can attach a single tab (with
optional widgets) to an **existing** `pageLayout` referenced by
`pageLayoutUniversalIdentifier`. The parent layout can be standard, from
the same app, or from another app — mirroring how `defineField`
references an object via `objectUniversalIdentifier`.

This complements `definePageLayout`: use `definePageLayout` when you own
the entire layout, use `definePageLayoutTab` when you only want to add
to one.

```ts
import { definePageLayoutTab, PageLayoutTabLayoutMode } from 'twenty-sdk/define';

export default definePageLayoutTab({
  universalIdentifier: 'b1b2b3b4-b5b6-4000-8000-000000000001',
  pageLayoutUniversalIdentifier: 'STANDARD-OR-OTHER-APP-PAGE-LAYOUT-UUID',
  title: 'Hello World',
  position: 1000,
  icon: 'IconWorld',
  layoutMode: PageLayoutTabLayoutMode.CANVAS,
  widgets: [/* ... */],
});
```

## Changes

- **twenty-shared**: new top-level `pageLayoutTabs:
PageLayoutTabManifest[]` on `Manifest`, optional
`pageLayoutUniversalIdentifier` on `PageLayoutTabManifest`, new
`SyncableEntity.PageLayoutTab`.
- **twenty-sdk**:
  - new `definePageLayoutTab` + `PageLayoutTabConfig` exports;
- manifest extraction wiring (`TargetFunction.DefinePageLayoutTab`,
`ManifestEntityKey.PageLayoutTabs`);
  - dev-mode label/state for the new entity;
- CLI scaffold (`getPageLayoutTabBaseFile`) + unit tests for `npx
twenty-cli add`.
- **twenty-server**: convert top-level `pageLayoutTabs` (and their
widgets) into universal flat entities in
`computeApplicationManifestAllUniversalFlatEntityMaps`. Cross-app FK
validation on `pageLayoutUniversalIdentifier` is already handled by the
existing `FlatPageLayoutTab` validator.
- **docs**: new `definePageLayoutTab` accordion in `apps/layout.mdx`
with usage example and guidance vs `definePageLayout`.
- **CI / rich-app fixture**: `extra-tab.page-layout-tab.ts` exercises
the new flow with a front-component widget; `expected-manifest.ts` and
`manifest.tests.ts` updated.

packages/twenty-shared/src/application/enums/syncable-entities.enum.ts

@@ -9,4 +9,5 @@ export enum SyncableEntity {
   View = 'view',
   NavigationMenuItem = 'navigationMenuItem',
   PageLayout = 'pageLayout',
+  PageLayoutTab = 'pageLayoutTab',
 }

packages/twenty-shared/src/application/manifestType.ts

@@ -6,7 +6,10 @@ import { type FrontComponentManifest } from './frontComponentManifestType';
 import { type LogicFunctionManifest } from './logicFunctionManifestType';
 import { type NavigationMenuItemManifest } from './navigationMenuItemManifestType';
 import { type ObjectManifest } from './objectManifestType';
-import { type PageLayoutManifest } from './pageLayoutManifestType';
+import {
+  type PageLayoutManifest,
+  type PageLayoutTabManifest,
+} from './pageLayoutManifestType';
 import { type RoleManifest } from './roleManifestType';
 import { type SkillManifest } from './skillManifestType';
 import { type ViewManifest } from './viewManifestType';
@@ -24,4 +27,5 @@ export type Manifest = {
   views: ViewManifest[];
   navigationMenuItems: NavigationMenuItemManifest[];
   pageLayouts: PageLayoutManifest[];
+  pageLayoutTabs: PageLayoutTabManifest[];
 };

packages/twenty-shared/src/application/pageLayoutManifestType.ts

@@ -21,6 +21,7 @@ export type PageLayoutTabManifest = SyncableEntityOptions & {
   icon?: string;
   layoutMode?: PageLayoutTabLayoutMode;
   widgets?: PageLayoutWidgetManifest[];
+  pageLayoutUniversalIdentifier?: string;
 };
 
 export type PageLayoutManifest = SyncableEntityOptions & {