Keep current rows dimmed during paginated list page transitions

[tom2drum]

Description

Design and ship the page-transition state for paginated lists: when the user moves to another page of the same list (same filters and sorting, different cursor), the current rows should stay on screen in a dimmed, non-interactive state until the next page arrives, then be replaced in one commit. Today every page change renders a skeleton pass instead.

This is the behaviour change that was split out of #3697. The rest of that refactor (URL as the single source of truth, one request per action, cached pages shown without a skeleton) lands without it; skeletons remain the treatment for page changes until the design here is agreed.

Details

• The plumbing is already in place but inert: the paginated query hook exposes an isTransitioning flag that is always false, and the shared list wrapper (DataList) accepts an isTransitioning prop that dims its body with the disabled-control opacity, disables pointer events and sets aria-busy. Every migrated list already passes the flag through.
• Flipping the behaviour means the hook's placeholderData keeps the previous page's data when the new query key differs from the previous one only by page / cursor, and falls back to the caller's stub otherwise (first load, filter or sort change). isInitialLoading stays true only in the stub case; pagination.isLoading stays true in both so the pagination buttons disable.
• Row keys already drop their index suffix for real data, so the previous rows keep their identity during the transition and the new rows replace them in a single commit.

Expected Behavior

• Design specifies the dimmed treatment: opacity value or token, whether a fade transition is used and its duration.
• Product confirms that skeletons stay for first load and filter / sort changes only.
• During a page transition the pagination buttons are disabled while the sticky table header and sorting controls remain usable.
• The socket-driven "new items" notice on page 1 keeps prepending as it does today.
• Unit spec covers: page change keeps previous rows with isTransitioning true; filter change and first load render the stub with isInitialLoading true; "Prev" onto a cached page sets neither flag.
• Playwright screenshot baseline for the transition state of DataList.