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/components/ChannelPage.tsx

@@ -398,7 +398,6 @@ export default function ChannelPage({
             setView={setView}
             canRead={me.can_read}
             isDemo={me.is_demo}
-            onOpenWizard={() => {}}
             ytSearch={null}
             onYtSearch={() => {}}
             onExitYtSearch={() => {}}