fix: unify login footer layout

[Willxup]

Summary

• reuse the shared ImageSilo footer on the login page
• structure the login page as a dynamic-viewport flex layout so the footer stays within the viewport
• compact vertical spacing on shorter screens while preserving natural scrolling when content truly cannot fit
• remove the unintended internal login-page scrollbar caused by the overflow axis computation and oversized background pseudo-element
• cover footer presence plus root-page and login-container overflow in browser tests

Verification

• make check
• make e2e (desktop and mobile Chromium)
• manual local preview confirmed the root page and .login-page have matching client and scroll heights