refactor(channels): scope module state in ChannelsProvider

Lift the Channels manager's shared local state (name filter, status chip,
active tab, column-reset token, focus-a-channel intent) out of App into a
split-context ChannelsProvider, nested inside NavigationProvider so its
focusChannel/goToFullHistory actions can drive navigation. The Channels page,
the header search box + sync chip, the feed filter sidebar, and the
notifications inbox all read it via useChannels()/useChannelsActions() instead
of props drilled through App. Channels also internalises onViewChannel /
onFilterByTag / onFocusChannel (nav + feed-filters straight from providers) and
takes only { me, onOpenWizard } now. App loses five useStates, the focusChannel
helper, the focus-clear effect, and a large block of per-page prop plumbing.
(Platform S6b.2)

frontend/src/components/NotificationsPanel.tsx

@@ -23,6 +23,7 @@ import { channelYouTubeUrl, relativeFromMs, relativeTime } from "../lib/format";
 import { LEVEL_STYLE } from "./Toaster";
 import { PageToolbar } from "./PageShell";
 import { useNavigationActions } from "./NavigationProvider";
+import { useChannelsActions } from "./ChannelsProvider";
 import { useFeedFilters, useFeedFiltersActions } from "./FeedFiltersProvider";
 
 // The single notification center. "System" = durable, server-backed notifications (inbox
@@ -31,15 +32,14 @@ import { useFeedFilters, useFeedFiltersActions } from "./FeedFiltersProvider";
 // there's one indicator and one place to look. Transient toasts still pop via the Toaster.
 const POLL_MS = 8000;
 
-export default function NotificationsPanel({
-  onFocusChannel,
-}: {
-  onFocusChannel: (name: string) => void;
-}) {
+export default function NotificationsPanel() {
   const { t } = useTranslation();
   const { setPage } = useNavigationActions();
   const filters = useFeedFilters();
   const { setFilters } = useFeedFiltersActions();
+  // "Focus this channel in the manager" (from a new-subscription notification) lives in
+  // ChannelsProvider — read it here and pass it down to the notification rows.
+  const { focusChannel: onFocusChannel } = useChannelsActions();
   const qc = useQueryClient();
   const q = useLiveQuery<{ items: AppNotification[]; total: number }>(
     ["notifications"],