refactor(nav): lift navigation + history into NavigationProvider

Move page/channelView/ytSearch and the history/popstate/title effects out of the
App god component into a new NavigationProvider (split state/actions contexts so
the memo'd VideoCard doesn't re-render on a page/channel switch). App consumes it
via useNavigation/useNavigationActions but still passes the props down for now;
the consumer migration that removes the prop-drilling follows.

frontend/src/main.tsx

@@ -3,6 +3,7 @@ import { createRoot } from "react-dom/client";
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 import ErrorBoundary from "./components/ErrorBoundary";
 import { ConfirmProvider } from "./components/ConfirmProvider";
+import { NavigationProvider } from "./components/NavigationProvider";
 import "./i18n";
 import "./index.css";
 
@@ -35,7 +36,9 @@ const root =
     <QueryClientProvider client={queryClient}>
       <ErrorBoundary>
         <ConfirmProvider>
-          <App />
+          <NavigationProvider>
+            <App />
+          </NavigationProvider>
         </ConfirmProvider>
       </ErrorBoundary>
     </QueryClientProvider>