Fix search bar disappearing when no products match query

[285729101]

Fixes antiwork#3262

The search bar and tables were conditionally rendered based on current products.length, so when a search returned zero results the search bar would vanish and the table's useEffect (which triggers Inertia reloads on query change) would stop running. The user gets stuck with no way to search again.

The fix uses useRef to remember whether products/memberships existed on initial page load. The search bar stays visible as long as there were initial items or there's an active query, and the table components stay mounted so their search effects keep firing. No changes needed in the table components themselves since their hooks run before the early return.

Added a system test that searches for a nonexistent product, verifies the search bar persists, then clears the search and confirms products reappear.