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-docs/developers/extend/apps/layout.mdx

@@ -13,6 +13,7 @@ Layout entities control how your app surfaces inside Twenty's UI — what lives
 | **View** | A saved list configuration for an object — visible fields, order, filters, groups | `defineView` |
 | **Navigation Menu Item** | An entry in the left sidebar that links to a view or an external URL | `defineNavigationMenuItem` |
 | **Page Layout** | The tabs and widgets that make up a record's detail page | `definePageLayout` |
+| **Page Layout Tab** | A standalone tab attached to an existing page layout (standard or your own app's) | `definePageLayoutTab` |
 
 Views, navigation items, and page layouts reference each other by `universalIdentifier`:
 
@@ -127,5 +128,51 @@ Key points:
 - Each `widget` inside a tab can render a front component, a relation list, or other built-in widget types.
 - `position` on tabs controls their order. Use higher values (e.g., 50) to place custom tabs after built-in ones.
 
+</Accordion>
+<Accordion title="definePageLayoutTab" description="Add a tab to an existing page layout">
+
+`definePageLayoutTab` lets your app attach a single tab — with optional widgets — to an **existing** page layout. The most common use case is adding a custom tab (for example, an analytics or AI summary tab) to one of Twenty's built-in record pages, or to a page layout your own app already ships.
+
+The targeted page layout must be either a **standard** Twenty page layout or one defined by **your own app**; cross-app references to page layouts owned by another installed app are not supported today.
+
+```ts src/page-layouts/example-extra-tab.ts
+import {
+  definePageLayoutTab,
+  PageLayoutTabLayoutMode,
+} from 'twenty-sdk/define';
+import { HELLO_WORLD_FRONT_COMPONENT_UNIVERSAL_IDENTIFIER } from '../front-components/hello-world';
+
+const COMPANY_RECORD_PAGE_LAYOUT_UNIVERSAL_IDENTIFIER =
+  '20202020-ab01-4001-8001-c0aba11c0100';
+
+export default definePageLayoutTab({
+  universalIdentifier: 'b1b2b3b4-b5b6-4000-8000-000000000001',
+  pageLayoutUniversalIdentifier:
+    COMPANY_RECORD_PAGE_LAYOUT_UNIVERSAL_IDENTIFIER,
+  title: 'Hello World',
+  position: 1000,
+  icon: 'IconWorld',
+  layoutMode: PageLayoutTabLayoutMode.CANVAS,
+  widgets: [
+    {
+      universalIdentifier: 'b1b2b3b4-b5b6-4000-8000-000000000002',
+      title: 'Hello World',
+      type: 'FRONT_COMPONENT',
+      configuration: {
+        configurationType: 'FRONT_COMPONENT',
+        frontComponentUniversalIdentifier:
+          HELLO_WORLD_FRONT_COMPONENT_UNIVERSAL_IDENTIFIER,
+      },
+    },
+  ],
+});
+```
+
+Key points:
+- `pageLayoutUniversalIdentifier` is **required** when using `definePageLayoutTab` and must point to a page layout that already exists at install time (standard or your app's). When the parent page layout is missing, installation fails with a clear validation error.
+- `widgets` are scoped to this tab only — they reference front components, views, etc. exactly like widgets defined inline in `definePageLayout`.
+- `position` controls ordering against existing tabs on the targeted layout. Pick a value that places your tab where you want it relative to built-in tabs.
+- Use this instead of `definePageLayout` when you only want to **add** to an existing layout. Use `definePageLayout` when you own the entire layout (typically a `RECORD_PAGE` for an object you ship in your app, or a `STANDALONE_PAGE`).
+
 </Accordion>
 </AccordionGroup>