Replace sign-in mocked metadata with hardcoded BackgroundMock (#20308)

## Summary

When the user is logged out, we render the auth modal on top of a sample
table to make the empty page feel alive. So far this was achieved by
**loading a full set of mocked object / field / view / navigation-menu
metadata into the runtime metadata store** and then mounting the real
`RecordTable` and `AppNavigationDrawer` behind the modal. This had a few
downsides:

- Significant bundle weight pulled in for unauthenticated users (mocked
GraphQL fixtures + the real `RecordTable` virtualization stack).
- Plenty of code paths that had to know about the "showAuthModal" case
(`useRecordIndexTableQuery`, `useTriggerInitialRecordTableDataLoad`,
`MainContextStoreProvider`, `IsMinimalMetadataReadyEffect`...).
- Any change to metadata-store internals or to the record-table runtime
risked breaking the logged-out background.

This PR replaces the entire flow with a small, self-contained
`BackgroundMock` component tree that **does not consume any metadata**
and **does not load any mocked metadata at runtime**.

### What changed

- New module under `sign-in-background-mock`:
- `BackgroundMockPage` + `BackgroundMockViewBar` + `BackgroundMockTable`
+ `BackgroundMockTableRow` render a hardcoded "Companies" table that
visually mirrors the real one.
- `BackgroundMockNavigationDrawer` renders a hardcoded sidebar with
People / Companies / Opportunities / Tasks / Notes (with their standard
colors).
- Hardcoded constants in `BackgroundMockCompanies.ts`,
`BackgroundMockColumns.ts`, `BackgroundMockNavigationItems.ts`.
- `MinimalMetadataLoadEffect` no longer calls `loadMockedMetadataAtomic`
for unauthenticated users — it just doesn't load anything.
- `IsMinimalMetadataReadyEffect` now reports ready immediately when
there is no access token pair, so the skeleton loader doesn't hang
waiting for metadata that will never come.
- `MainContextStoreProvider`, `useRecordIndexTableQuery`, and
`useTriggerInitialRecordTableDataLoad` drop their `showAuthModal`
branches — the real `RecordTable` is no longer mounted behind the modal.
- `DefaultLayout` and `NotFound` now lazily load `BackgroundMockPage` /
`BackgroundMockNavigationDrawer` instead of the deleted
`SignInBackgroundMockPage` / `SignInAppNavigationDrawerMock`.
- Removed: `SignInBackgroundMockPage`, `SignInBackgroundMockContainer`,
`SignInBackgroundMockContainerEffect`, `SignInAppNavigationDrawerMock`,
`SignInBackgroundMockColumnDefinitions`,
`SignInBackgroundMockCompanies`, `SignInBackgroundMockViewFields`.

`useLoadMockedMetadata` and `preloadMockedMetadata` are kept on purpose:
Storybook decorators (`ObjectMetadataItemsDecorator`,
`WorkflowStepDecorator`) still rely on the mocked metadata fixtures, but
**production** unauthenticated runtime no longer touches them.

### Visual parity

Side-by-side at 1440×900 on `/sign-in`:

**Before** (loads mocked metadata + real RecordTable):

![before](https://github.com/user-attachments/assets/before-placeholder)

**After** (purely hardcoded BackgroundMock):

![after](https://github.com/user-attachments/assets/after-placeholder)

## Test plan

- [ ] `npx nx typecheck twenty-front` ✅ (passes locally)
- [ ] `npx nx lint:diff-with-main twenty-front` ✅ (oxlint + prettier
clean)
- [ ] `npx jest useRecordIndexTableQuery` ✅
- [ ] Manually verify `/sign-in` renders the table + nav drawer behind
the modal
- [ ] Manually verify `/not-found` still renders the background
- [ ] Verify CI: storybook, unit tests, e2e tests

packages/twenty-front/src/modules/context-store/components/MainContextStoreProvider.tsx

@@ -3,7 +3,6 @@ import { metadataStoreState } from '@/metadata-store/states/metadataStoreState';
 import { useIsSettingsPage } from '@/navigation/hooks/useIsSettingsPage';
 import { useLastVisitedView } from '@/navigation/hooks/useLastVisitedView';
 import { objectMetadataItemsSelector } from '@/object-metadata/states/objectMetadataItemsSelector';
-import { useShowAuthModal } from '@/ui/layout/hooks/useShowAuthModal';
 import { useAtomFamilyStateValue } from '@/ui/utilities/state/jotai/hooks/useAtomFamilyStateValue';
 import { useAtomStateValue } from '@/ui/utilities/state/jotai/hooks/useAtomStateValue';
 import { viewsSelector } from '@/views/states/selectors/viewsSelector';
@@ -38,8 +37,6 @@ const getViewId = (
   return undefined;
 };
 
-const SIGN_IN_BACKGROUND_OBJECT_NAME_PLURAL = 'companies';
-
 export const MainContextStoreProvider = () => {
   const location = useLocation();
   const isRecordIndexPage = isMatchingLocation(
@@ -49,15 +46,10 @@ export const MainContextStoreProvider = () => {
   const isRecordShowPage = isMatchingLocation(location, AppPath.RecordShowPage);
   const isStandalonePage = isMatchingLocation(location, AppPath.PageLayoutPage);
   const isSettingsPage = useIsSettingsPage();
-  const showAuthModal = useShowAuthModal();
 
-  const objectNamePluralFromParams = useParams().objectNamePlural ?? '';
+  const objectNamePlural = useParams().objectNamePlural ?? '';
   const objectNameSingular = useParams().objectNameSingular ?? '';
 
-  const objectNamePlural = showAuthModal
-    ? SIGN_IN_BACKGROUND_OBJECT_NAME_PLURAL
-    : objectNamePluralFromParams;
-
   const [searchParams] = useSearchParams();
   const viewIdQueryParamRaw = searchParams.get('viewId');
 
@@ -120,8 +112,7 @@ export const MainContextStoreProvider = () => {
     (isRecordIndexPage ||
       isRecordShowPage ||
       isStandalonePage ||
-      isSettingsPage ||
-      showAuthModal) &&
+      isSettingsPage) &&
     metadataStore.status === 'up-to-date';
 
   if (!shouldComputeContextStore) {