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;