feat(channels): clearer error when a YouTube action lacks the scope

api: HttpError now carries the server's detail. Channels sync/backfill/unsubscribe
now detect a 403 (no YouTube grant) and show a 'connect your YouTube account'
message with a Connect action that opens the onboarding wizard, instead of a vague
'failed' toast. Translated HU/EN/DE.

frontend/src/App.tsx

@@ -203,6 +203,7 @@ export default function App() {
               canWrite={meQuery.data!.can_write}
               statusFilter={channelFilter}
               setStatusFilter={setChannelFilter}
+              onOpenWizard={() => setWizardOpen(true)}
               onViewChannel={(id, name) => {
                 setFilters({ ...filters, channelId: id, channelName: name, show: "all" });
                 setPage("feed");