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-sdk/src/cli/commands/add.ts

@@ -15,6 +15,7 @@ import { getLogicFunctionBaseFile } from '@/cli/utilities/entity/entity-logic-fu
 import { getNavigationMenuItemBaseFile } from '@/cli/utilities/entity/entity-navigation-menu-item-template';
 import { getObjectBaseFile } from '@/cli/utilities/entity/entity-object-template';
 import { getPageLayoutBaseFile } from '@/cli/utilities/entity/entity-page-layout-template';
+import { getPageLayoutTabBaseFile } from '@/cli/utilities/entity/entity-page-layout-tab-template';
 import { getRecordPageLayoutBaseFile } from '@/cli/utilities/entity/entity-record-page-layout-template';
 import { getRoleBaseFile } from '@/cli/utilities/entity/entity-role-template';
 import { getAgentBaseFile } from '@/cli/utilities/entity/entity-agent-template';
@@ -189,6 +190,15 @@ export class EntityAddCommand {
         return { name, file };
       }
 
+      case SyncableEntity.PageLayoutTab: {
+        const name = await this.getEntityName(entity);
+
+        const file = getPageLayoutTabBaseFile({
+          name,
+        });
+        return { name, file };
+      }
+
       default:
         assertUnreachable(entity);
     }