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/Sidebar.tsx

@@ -14,6 +14,7 @@ import SidePanel from "./SidePanel";
 import PanelGroups, { type PanelItem } from "./PanelGroups";
 import ChannelPicker from "./ChannelPicker";
 import { useFeedFilters, useFeedFiltersActions } from "./FeedFiltersProvider";
+import { useChannelsActions } from "./ChannelsProvider";
 
 // Filter ids; display labels resolved at render time via t("sidebar.sort.<id>") etc.
 const DATE_PRESETS: { days: number; key: string }[] = [
@@ -63,14 +64,12 @@ function TagChip({
 export default function Sidebar({
   layout,
   setLayout,
-  onFocusChannel,
   isDemo = false,
   collapsed,
   onToggleCollapse,
 }: {
   layout: PanelLayout;
   setLayout: (l: PanelLayout) => void;
-  onFocusChannel: (name: string) => void;
   isDemo?: boolean;
   collapsed: boolean;
   onToggleCollapse: () => void;
@@ -78,6 +77,8 @@ export default function Sidebar({
   const { t } = useTranslation();
   const filters = useFeedFilters();
   const { setFilters } = useFeedFiltersActions();
+  // "Focus this channel in the manager" (from the tag manager here) lives in ChannelsProvider.
+  const { focusChannel: onFocusChannel } = useChannelsActions();
   const tagsQuery = useQuery({ queryKey: ["tags"], queryFn: api.tags });
   const tags = tagsQuery.data ?? [];