Remove twenty-ui reexport from the SDK and use twenty-ui directly (#22326)

## What & why

Removes the `twenty-sdk/ui` reexport. Apps now use Twenty UI by
installing
[`twenty-ui@1.0.0-alpha.1`](https://www.npmjs.com/package/twenty-ui/v/1.0.0-alpha.1)
from npm and importing its subpaths directly. The reexport re-exported
types that didn't resolve, forcing typecheck workarounds.

## Changes

- **twenty-sdk**: delete `src/ui/index.ts`, drop the `./ui` export,
remove it from the browser vite build, and rewire the CLI manifest-mock
to `twenty-ui` (`.css` falls through to the empty-CSS loader).
`twenty-ui` stays a devDependency for the CLI fixture tests.
- **Renderer + create-twenty-app template**: import from `twenty-ui`
subpaths; the template pins `twenty-ui@1.0.0-alpha.1`.
- **Docs**: new "Using Twenty UI components" section (install + subpath
imports + `useTheme()` for theme tokens), codex references, and the
cross-doc-contract validator.

The `twenty-for-twenty` / `twenty-slack` example apps are intentionally
left on `twenty-sdk/ui`: they consume the published SDK (which still
ships `./ui`), and `twenty-ui@1.0.0-alpha.1` requires react 19 + a
`monaco-editor` peer the react-18 apps can't satisfy. They migrate once
the SDK is republished.

packages/twenty-codex-plugin/scripts/validators/cross-doc-contracts.js

@@ -82,12 +82,12 @@ const assertFrontComponentGuidance = (fail) => {
     'defineFrontComponent',
     'Use `twenty-sdk/front-component`',
     'Use `twenty-client-sdk/core` or `twenty-client-sdk/metadata`',
-    'Use `twenty-sdk/ui` for Twenty UI components',
-    'Do not import from `twenty-ui` directly',
+    'twenty-ui@1.0.0-alpha.1',
+    'import Twenty UI components, icons, and theme tokens from its subpaths',
     'ThemeProvider',
-    'example-sources/twenty-ui-example.front-component.tsx',
+    'showcase/twenty-ui-example.front-component.tsx',
     'themeCssVariables',
-    'mocks `twenty-sdk/ui` during manifest extraction',
+    'mocks `twenty-ui` during manifest extraction',
     'A clean typecheck and sync is not runtime verification',
     'without a `FrontComponent error`',
     'hard refresh',