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-server/src/engine/core-modules/application/application-manifest/application-manifest-migration.service.ts

@@ -78,6 +78,7 @@ export class ApplicationManifestMigrationService {
       views: [],
       navigationMenuItems: [],
       pageLayouts: [],
+      pageLayoutTabs: [],
     };
 
     const now = new Date().toISOString();

packages/twenty-server/src/engine/core-modules/application/application-manifest/utils/compute-application-manifest-all-universal-flat-entity-maps.util.ts

@@ -397,5 +397,42 @@ export const computeApplicationManifestAllUniversalFlatEntityMaps = ({
     }
   }
 
+  for (const pageLayoutTabManifest of manifest.pageLayoutTabs ?? []) {
+    if (!isDefined(pageLayoutTabManifest.pageLayoutUniversalIdentifier)) {
+      throw new Error(
+        `Top-level pageLayoutTab "${pageLayoutTabManifest.universalIdentifier}" is missing required pageLayoutUniversalIdentifier`,
+      );
+    }
+
+    addUniversalFlatEntityToUniversalFlatEntityMapsThroughMutationOrThrow({
+      universalFlatEntity:
+        fromPageLayoutTabManifestToUniversalFlatPageLayoutTab({
+          pageLayoutTabManifest,
+          pageLayoutUniversalIdentifier:
+            pageLayoutTabManifest.pageLayoutUniversalIdentifier,
+          applicationUniversalIdentifier,
+          now,
+        }),
+      universalFlatEntityMapsToMutate:
+        allUniversalFlatEntityMaps.flatPageLayoutTabMaps,
+    });
+
+    for (const pageLayoutWidgetManifest of pageLayoutTabManifest.widgets ??
+      []) {
+      addUniversalFlatEntityToUniversalFlatEntityMapsThroughMutationOrThrow({
+        universalFlatEntity:
+          fromPageLayoutWidgetManifestToUniversalFlatPageLayoutWidget({
+            pageLayoutWidgetManifest,
+            pageLayoutTabUniversalIdentifier:
+              pageLayoutTabManifest.universalIdentifier,
+            applicationUniversalIdentifier,
+            now,
+          }),
+        universalFlatEntityMapsToMutate:
+          allUniversalFlatEntityMaps.flatPageLayoutWidgetMaps,
+      });
+    }
+  }
+
   return allUniversalFlatEntityMaps;
 };

packages/twenty-server/src/engine/core-modules/application/application-marketplace/utils/__tests__/resolve-manifest-asset-urls.util.spec.ts

@@ -25,6 +25,7 @@ const buildMinimalManifest = (
   views: [],
   navigationMenuItems: [],
   pageLayouts: [],
+  pageLayoutTabs: [],
 });
 
 describe('resolveManifestAssetUrls', () => {