Lift the Plex module's shared local state (scope / watch-state / sort / expanded
filters / search term / open-playlist hand-off) out of App into a split-context
PlexProvider with stable useCallback setters. PlexSidebar, PlexBrowse, and the
header search box read it via usePlex()/usePlexActions() instead of props drilled
through App; PlexBrowse is now prop-free. App loses six useStates + the filters
JSON memo/serialise helpers. (Platform S6b.2)