Let the sign-in screen scroll when its content overflows the viewport (#22397)

On the sign-in screen, when a step's content is taller than the viewport
(e.g. many workspaces to choose from), it grew past the fixed-height
background and overflowed the page.

Make the shared background scroll instead, so every sign-in step scrolls
when its content overflows and stays centered when it fits.

## Before


https://github.com/user-attachments/assets/0226daee-0cd9-454c-9f4b-257cfab61bfb


## After


https://github.com/user-attachments/assets/7a535099-f0f0-431b-86ad-9fa8121638db


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packages/twenty-front/src/pages/auth/SignInUpV2.tsx

@@ -51,6 +51,7 @@ const StyledBackground = styled.div`
   display: flex;
   flex-direction: column;
   height: 100dvh;
+  overflow-y: auto;
   width: 100%;
 `;