fix(front): suppress full-page skeleton inside auth modal (#19875)

## Summary

- Lazy auth-flow routes (`SignInUp`, `Invite`, `ResetPassword`,
`CreateWorkspace`, `CreateProfile`, `SyncEmails`, `InviteTeam`,
`PlanRequired`, `PlanRequiredSuccess`, `BookCallDecision`, `BookCall`)
render through `<Outlet/>` inside `<AuthModal>`. Their `LazyRoute`
`<Suspense>` fallback was the page-level `PageContentSkeletonLoader`, so
the two grey shimmer bars painted **inside the modal box** for a few
hundred ms while each chunk downloaded.
- `LazyRoute` now accepts an optional `fallback` prop (default
unchanged: the existing page skeleton). Every auth-modal route passes
`fallback={null}` so the modal stays empty until the lazy chunk resolves
instead of flashing the shimmer.
- `AuthModal`'s inner `StyledContent` gets a `min-height: 320px` so the
framer-motion `layout` animation doesn't rapidly resize the modal as
inner steps (loader → form → password → 2FA / workspace selection) swap.
The modal can still grow for taller steps; only the rapid jump is
removed.

## Why default-parameter syntax for `fallback`

`fallback ?? <LazyRouteFallback/>` would treat an explicit `null` as "no
value" and still render the default skeleton. Using a default parameter
(`fallback = <LazyRouteFallback/>`) preserves an explicit `null` because
defaults only kick in for `undefined`.

packages/twenty-front/src/modules/app/hooks/useCreateAppRouter.tsx

@@ -124,7 +124,7 @@ export const useCreateAppRouter = (
           <Route
             path={AppPath.SignInUp}
             element={
-              <LazyRoute>
+              <LazyRoute fallback={null}>
                 <SignInUp />
               </LazyRoute>
             }
@@ -132,7 +132,7 @@ export const useCreateAppRouter = (
           <Route
             path={AppPath.Invite}
             element={
-              <LazyRoute>
+              <LazyRoute fallback={null}>
                 <SignInUp />
               </LazyRoute>
             }
@@ -140,7 +140,7 @@ export const useCreateAppRouter = (
           <Route
             path={AppPath.ResetPassword}
             element={
-              <LazyRoute>
+              <LazyRoute fallback={null}>
                 <PasswordReset />
               </LazyRoute>
             }
@@ -148,7 +148,7 @@ export const useCreateAppRouter = (
           <Route
             path={AppPath.CreateWorkspace}
             element={
-              <LazyRoute>
+              <LazyRoute fallback={null}>
                 <CreateWorkspace />
               </LazyRoute>
             }
@@ -156,7 +156,7 @@ export const useCreateAppRouter = (
           <Route
             path={AppPath.CreateProfile}
             element={
-              <LazyRoute>
+              <LazyRoute fallback={null}>
                 <CreateProfile />
               </LazyRoute>
             }
@@ -164,7 +164,7 @@ export const useCreateAppRouter = (
           <Route
             path={AppPath.SyncEmails}
             element={
-              <LazyRoute>
+              <LazyRoute fallback={null}>
                 <SyncEmails />
               </LazyRoute>
             }
@@ -172,7 +172,7 @@ export const useCreateAppRouter = (
           <Route
             path={AppPath.InviteTeam}
             element={
-              <LazyRoute>
+              <LazyRoute fallback={null}>
                 <InviteTeam />
               </LazyRoute>
             }
@@ -180,7 +180,7 @@ export const useCreateAppRouter = (
           <Route
             path={AppPath.PlanRequired}
             element={
-              <LazyRoute>
+              <LazyRoute fallback={null}>
                 <ChooseYourPlan />
               </LazyRoute>
             }
@@ -188,7 +188,7 @@ export const useCreateAppRouter = (
           <Route
             path={AppPath.PlanRequiredSuccess}
             element={
-              <LazyRoute>
+              <LazyRoute fallback={null}>
                 <PaymentSuccess />
               </LazyRoute>
             }
@@ -196,7 +196,7 @@ export const useCreateAppRouter = (
           <Route
             path={AppPath.BookCallDecision}
             element={
-              <LazyRoute>
+              <LazyRoute fallback={null}>
                 <BookCallDecision />
               </LazyRoute>
             }
@@ -204,7 +204,7 @@ export const useCreateAppRouter = (
           <Route
             path={AppPath.BookCall}
             element={
-              <LazyRoute>
+              <LazyRoute fallback={null}>
                 <BookCall />
               </LazyRoute>
             }