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/main.tsx

@@ -7,6 +7,7 @@ import { NavigationProvider } from "./components/NavigationProvider";
 import { FeedFiltersProvider } from "./components/FeedFiltersProvider";
 import { PlaylistsProvider } from "./components/PlaylistsProvider";
 import { PlexProvider } from "./components/PlexProvider";
+import { ChannelsProvider } from "./components/ChannelsProvider";
 import "./i18n";
 import "./index.css";
 
@@ -43,7 +44,9 @@ const root =
             <FeedFiltersProvider>
               <PlaylistsProvider>
                 <PlexProvider>
-                  <App />
+                  <ChannelsProvider>
+                    <App />
+                  </ChannelsProvider>
                 </PlexProvider>
               </PlaylistsProvider>
             </FeedFiltersProvider>