Fix lobby room list scrolling

[coder13]

What changed

• constrain the lobby flex hierarchy to the available viewport height
• keep both room and user panels bounded within the Material UI Grid row
• add a Cypress regression that forces overflow in a short viewport and verifies real scroll movement

Root cause

The room list declared overflow-y: auto, but flex and Grid ancestors remained content-sized. The room panel therefore grew beyond the viewport instead of giving the list a bounded scroll area; global page overflow then clipped the remaining rooms.

User impact

Users can scroll through all public and private rooms again, including in short and mobile-sized viewports.

Validation

• yarn workspace letscube-client lint (passes; existing warnings only)
• yarn workspace letscube-client test:ci --runInBand (97/97)
• yarn workspace letscube-client build
• yarn cypress:run --spec cypress/e2e/local_stack.cy.js (11/11)
• git diff --check
• full committed diff reviewed with no findings