Shared presentation decks no longer render as a 24px sliver

Shared decks using auto-fit sizing could measure their container before it had been laid out — especially inside the anonymous share shell with its marketing header, banner, and cookie bar — and get pinned to a collapsed ~24px height for the rest of the session. Sizing is now measured more reliably, viewport-relative decks are deterministically pinned to a single screen from the start, and the heal path survives re-renders (such as a comment overlay arriving asynchronously) instead of clinging to a stale, detached element.