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/components/Sidebar.tsx

@@ -13,6 +13,7 @@ import SavedViewsWidget from "./SavedViewsWidget";
 import SidePanel from "./SidePanel";
 import PanelGroups, { type PanelItem } from "./PanelGroups";
 import ChannelPicker from "./ChannelPicker";
+import { useFeedFilters, useFeedFiltersActions } from "./FeedFiltersProvider";
 
 // Filter ids; display labels resolved at render time via t("sidebar.sort.<id>") etc.
 const DATE_PRESETS: { days: number; key: string }[] = [
@@ -60,8 +61,6 @@ function TagChip({
 }
 
 export default function Sidebar({
-  filters,
-  setFilters,
   layout,
   setLayout,
   onFocusChannel,
@@ -69,8 +68,6 @@ export default function Sidebar({
   collapsed,
   onToggleCollapse,
 }: {
-  filters: FeedFilters;
-  setFilters: (f: FeedFilters) => void;
   layout: PanelLayout;
   setLayout: (l: PanelLayout) => void;
   onFocusChannel: (name: string) => void;
@@ -79,6 +76,8 @@ export default function Sidebar({
   onToggleCollapse: () => void;
 }) {
   const { t } = useTranslation();
+  const filters = useFeedFilters();
+  const { setFilters } = useFeedFiltersActions();
   const tagsQuery = useQuery({ queryKey: ["tags"], queryFn: api.tags });
   const tags = tagsQuery.data ?? [];