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/testing/decorators/MockedMetadataLoadEffect.tsx

@@ -0,0 +1,31 @@
+import { useLoadMockedMetadata } from '@/metadata-store/hooks/useLoadMockedMetadata';
+import { isMinimalMetadataReadyState } from '@/metadata-store/states/isMinimalMetadataReadyState';
+import { metadataStoreState } from '@/metadata-store/states/metadataStoreState';
+import { useAtomFamilyStateValue } from '@/ui/utilities/state/jotai/hooks/useAtomFamilyStateValue';
+import { useSetAtomState } from '@/ui/utilities/state/jotai/hooks/useSetAtomState';
+import { useEffect } from 'react';
+
+export const MockedMetadataLoadEffect = () => {
+  const { loadMockedMetadataAtomic } = useLoadMockedMetadata();
+  const metadataStore = useAtomFamilyStateValue(
+    metadataStoreState,
+    'objectMetadataItems',
+  );
+  const setIsMinimalMetadataReady = useSetAtomState(
+    isMinimalMetadataReadyState,
+  );
+
+  useEffect(() => {
+    void loadMockedMetadataAtomic();
+  }, [loadMockedMetadataAtomic]);
+
+  useEffect(() => {
+    if (metadataStore.status === 'up-to-date') {
+      setIsMinimalMetadataReady(true);
+    } else {
+      setIsMinimalMetadataReady(false);
+    }
+  }, [metadataStore.status, setIsMinimalMetadataReady]);
+
+  return null;
+};

packages/twenty-front/src/testing/decorators/PageDecorator.tsx

@@ -16,8 +16,7 @@ import { ApolloCoreClientMockedProvider } from '@/object-metadata/hooks/__mocks_
 import { DefaultLayout } from '@/ui/layout/page/components/DefaultLayout';
 import { MinimalMetadataGater } from '@/metadata-store/components/MinimalMetadataGater';
 import { UserMetadataProviderInitialEffect } from '@/metadata-store/effect-components/UserMetadataProviderInitialEffect';
-import { IsMinimalMetadataReadyEffect } from '@/metadata-store/effect-components/IsMinimalMetadataReadyEffect';
-import { MinimalMetadataLoadEffect } from '@/metadata-store/effect-components/MinimalMetadataLoadEffect';
+import { MockedMetadataLoadEffect } from '~/testing/decorators/MockedMetadataLoadEffect';
 import { jotaiStore } from '@/ui/utilities/state/jotai/jotaiStore';
 import { useState } from 'react';
 import { ClientConfigProvider } from '~/modules/client-config/components/ClientConfigProvider';
@@ -87,8 +86,7 @@ const Providers = () => {
             <ClientConfigProviderEffect />
             <ClientConfigProvider>
               <UserMetadataProviderInitialEffect />
-              <MinimalMetadataLoadEffect />
-              <IsMinimalMetadataReadyEffect />
+              <MockedMetadataLoadEffect />
               <WorkspaceProviderEffect />
               <MinimalMetadataGater>
                 <ApolloCoreClientMockedProvider>