feat(onboarding): auto-import subscriptions after read grant + empty-feed guidance

After read access is granted the wizard now imports the user's YouTube
subscriptions automatically (with a "Building your feed…" progress state), so a
new user lands on a populated feed — channels already in the shared catalog show
up instantly, new ones backfill in the background. The empty feed now prompts
users without read access to set up via the wizard instead of a bare message.

frontend/src/App.tsx

@@ -179,7 +179,13 @@ export default function App() {
           ) : page === "stats" && meQuery.data!.role === "admin" ? (
             <Stats />
           ) : (
-            <Feed filters={filters} setFilters={setFilters} view={view} />
+            <Feed
+              filters={filters}
+              setFilters={setFilters}
+              view={view}
+              canRead={meQuery.data!.can_read}
+              onOpenWizard={() => setWizardOpen(true)}
+            />
           )}
         </main>
       </div>