Add smooth page transitions to onboarding v2 (#22392)

## Before


https://github.com/user-attachments/assets/d2fcd5ce-7e34-4f07-9a52-cac8acdc37cd

## After


https://github.com/user-attachments/assets/5c245949-cff9-41b2-802d-3deeb562efa6


On a full-page load of a v2 onboarding URL (the post-signup
workspace-subdomain redirect), Lingui's `I18nProvider` renders `null`
until the locale chunk async-activates, so the app is blank for ~2s
before the verify step appears. Steps also hard-cut and flashed a loader
between each other.

- Show a pulsing-logo loader until the locale activates (a gate above
`I18nProvider`), scoped to onboarding v2 paths so every other page is
unchanged.
- Cross-fade between steps and preload their chunks on entry, so
navigating never flashes the loader.

Frontend-only; i18n loading itself is untouched.

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packages/twenty-front/src/modules/auth/constants/OnboardingV2Paths.ts

@@ -0,0 +1,12 @@
+import { AppPath } from 'twenty-shared/types';
+
+export const ONBOARDING_V2_PATHS = [
+  AppPath.SignInUpV2,
+  AppPath.VerifyV2,
+  AppPath.WorkspaceActivationV2,
+  AppPath.CreateProfileV2,
+  AppPath.SyncEmailsV2,
+  AppPath.InstallAppsV2,
+  AppPath.InviteTeamV2,
+  AppPath.PlanRequiredV2,
+];