Render only the visible rows via @tanstack/react-virtual (useVirtualizer
against the app's <main> scroll container, per-row dynamic measurement),
so the DOM no longer accumulates every loaded card. A ResizeObserver keeps
the responsive grid column count in sync and chunks cards into virtualized
rows; the list view virtualizes single-card rows. Infinite scroll now
triggers from the virtual range and pages via next_cursor instead of
offset; feed/count drops its unused paging args.
