refactor(feed-view): move the feed view mode into FeedViewProvider

Lift the feed's view-mode state (server-synced: persists on pick, adopted from
prefs on login) out of App into FeedViewProvider. App reads it via useFeedView()
to hand to Feed/ChannelPage; the view adopt lines leave App's big prefs-adopt
effect. Removes another App-owned cross-cutting state ahead of the render flip.
(Platform S6b.3)

frontend/src/main.tsx

@@ -8,6 +8,7 @@ import { FeedFiltersProvider } from "./components/FeedFiltersProvider";
 import { PlaylistsProvider } from "./components/PlaylistsProvider";
 import { PlexProvider } from "./components/PlexProvider";
 import { ChannelsProvider } from "./components/ChannelsProvider";
+import { FeedViewProvider } from "./components/FeedViewProvider";
 import { WizardProvider } from "./components/WizardProvider";
 import "./i18n";
 import "./index.css";
@@ -46,9 +47,11 @@ const root =
               <PlaylistsProvider>
                 <PlexProvider>
                   <ChannelsProvider>
-                    <WizardProvider>
-                      <App />
-                    </WizardProvider>
+                    <FeedViewProvider>
+                      <WizardProvider>
+                        <App />
+                      </WizardProvider>
+                    </FeedViewProvider>
                   </ChannelsProvider>
                 </PlexProvider>
               </PlaylistsProvider>