refactor(plex): scope module state in PlexProvider

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)

frontend/src/main.tsx

@@ -6,6 +6,7 @@ import { ConfirmProvider } from "./components/ConfirmProvider";
 import { NavigationProvider } from "./components/NavigationProvider";
 import { FeedFiltersProvider } from "./components/FeedFiltersProvider";
 import { PlaylistsProvider } from "./components/PlaylistsProvider";
+import { PlexProvider } from "./components/PlexProvider";
 import "./i18n";
 import "./index.css";
 
@@ -41,7 +42,9 @@ const root =
           <NavigationProvider>
             <FeedFiltersProvider>
               <PlaylistsProvider>
-                <App />
+                <PlexProvider>
+                  <App />
+                </PlexProvider>
               </PlaylistsProvider>
             </FeedFiltersProvider>
           </NavigationProvider>