refactor(wizard): move onboarding wizard state into WizardProvider

Lift the onboarding wizard's open state, first-login auto-open effect, and the
wizard render itself out of App into WizardProvider. Feed, Channels, and
SettingsPanel trigger it via useWizard() instead of an onOpenWizard prop; App
and ChannelPage drop the prop plumbing. First step of S6b.3 — removing the
per-module props that block a uniform registry render. (Platform S6b.3)

frontend/src/main.tsx

@@ -8,6 +8,7 @@ import { FeedFiltersProvider } from "./components/FeedFiltersProvider";
 import { PlaylistsProvider } from "./components/PlaylistsProvider";
 import { PlexProvider } from "./components/PlexProvider";
 import { ChannelsProvider } from "./components/ChannelsProvider";
+import { WizardProvider } from "./components/WizardProvider";
 import "./i18n";
 import "./index.css";
 
@@ -45,7 +46,9 @@ const root =
               <PlaylistsProvider>
                 <PlexProvider>
                   <ChannelsProvider>
-                    <App />
+                    <WizardProvider>
+                      <App />
+                    </WizardProvider>
                   </ChannelsProvider>
                 </PlexProvider>
               </PlaylistsProvider>