fix(setup): hide YouTube/Google affordances when Google sign-in isn't configured

- /api/me exposes instance-wide google_enabled. The onboarding nudge no longer auto-opens, and
  Settings hides the YouTube-access section + the 'Connect Google' option, when the instance has
  no Google OAuth configured (e.g. an email+password-only self-host). A linked account still shows
  its 'Connected' status.

frontend/src/lib/api.ts

@@ -11,6 +11,7 @@ export interface Me {
   is_demo: boolean;
   has_google: boolean;
   has_password: boolean;
+  google_enabled: boolean;
   can_read: boolean;
   can_write: boolean;
   pending_invites: number;

frontend/src/lib/onboarding.ts

@@ -15,9 +15,16 @@
 export const ONBOARD_ACTIVE = "subfeed.onboarding.active";
 export const ONBOARD_DISMISSED = "subfeed.onboarding.dismissed";
 
-export function shouldAutoOpenOnboarding(me: { can_read: boolean; is_demo?: boolean }): boolean {
+export function shouldAutoOpenOnboarding(me: {
+  can_read: boolean;
+  is_demo?: boolean;
+  google_enabled?: boolean;
+}): boolean {
   // The shared demo account can never connect YouTube, so never nudge it to.
   if (me.is_demo) return false;
+  // YouTube connect needs Google OAuth configured on this instance. A self-host instance may run
+  // email+password only — then there's nothing to connect, so don't nudge.
+  if (me.google_enabled === false) return false;
   if (sessionStorage.getItem(ONBOARD_ACTIVE)) return true;
   return !me.can_read && !localStorage.getItem(ONBOARD_DISMISSED);
 }