fix(demo): graceful UX when YouTube actions are unavailable

The shared demo account no longer hits YouTube affordances: the onboarding
wizard never opens (or renders) for it, the empty-feed prompt nudges into
the shared library instead of the connect wizard, and the browser-facing
/auth/upgrade redirects the demo home instead of returning a raw 403 JSON.

frontend/src/App.tsx

@@ -291,13 +291,14 @@ export default function App() {
               setFilters={setFilters}
               view={view}
               canRead={meQuery.data!.can_read}
+              isDemo={meQuery.data!.is_demo}
               onOpenWizard={() => setWizardOpen(true)}
             />
           )}
           </main>
         </div>
       </div>
-      {wizardOpen && meQuery.data && (
+      {wizardOpen && meQuery.data && !meQuery.data.is_demo && (
         <OnboardingWizard me={meQuery.data} onClose={() => setWizardOpen(false)} />
       )}
       {aboutOpen && (