feat(filters): scope global feed filters in FeedFiltersProvider

Lift the feed's filter god-state (per-account persistence, the account-load
effect, share-link capture) out of App into a split-context provider mirroring
NavigationProvider. Sidebar/Header/NotificationsPanel read it via hooks instead
of prop-drilling; <Feed> keeps its filters prop since ChannelPage renders it with
its own channel-scoped state.

frontend/src/main.tsx

@@ -4,6 +4,7 @@ import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 import ErrorBoundary from "./components/ErrorBoundary";
 import { ConfirmProvider } from "./components/ConfirmProvider";
 import { NavigationProvider } from "./components/NavigationProvider";
+import { FeedFiltersProvider } from "./components/FeedFiltersProvider";
 import "./i18n";
 import "./index.css";
 
@@ -37,7 +38,9 @@ const root =
       <ErrorBoundary>
         <ConfirmProvider>
           <NavigationProvider>
-            <App />
+            <FeedFiltersProvider>
+              <App />
+            </FeedFiltersProvider>
           </NavigationProvider>
         </ConfirmProvider>
       </ErrorBoundary>