Discover and libraries now fetch only the servers that came online since
the last pass and merge the results; already-loaded servers are not
refetched. Both providers also own their online-servers listener
(registered in the constructor, removed in dispose), so profile switches
no longer leak listeners that kept disposed providers refetching.
- Skip redundant notifyListeners in HiddenLibrariesProvider init when
set is empty (empty→empty is a no-op for consumers)
- Track last-seen hidden keys in DiscoverScreen to avoid reloading
content when the listener fires without an actual change
- Track previous online server set in MultiServerProvider; only re-check
DVR availability when a new server comes online
- Remove dead content-loading pipeline from libraries_screen (sorts,
pagination, filters, items) that was never rendered
- Move /media/providers fetch into PlexClient.create() init, replacing
lazy EPG cache and providing libraries including shared items
edde746