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.

backend/app/routes/me.py

@@ -2,7 +2,14 @@ from fastapi import APIRouter, BackgroundTasks, Depends, HTTPException, Request
 from sqlalchemy import func, select
 from sqlalchemy.orm import Session
 
-from app.auth import current_user, has_read_scope, has_write_scope, is_allowed, purge_user
+from app.auth import (
+    current_user,
+    google_enabled,
+    has_read_scope,
+    has_write_scope,
+    is_allowed,
+    purge_user,
+)
 from app.db import get_db
 from app.models import Invite, User
 
@@ -80,6 +87,9 @@ def get_me(
         "is_demo": user.is_demo,
         "has_google": user.google_sub is not None,
         "has_password": user.password_hash is not None,
+        # Instance-wide: whether Google sign-in / YouTube connect is available at all (the UI hides
+        # YouTube-access affordances and the onboarding nudge when it isn't configured).
+        "google_enabled": google_enabled(db),
         "can_read": has_read_scope(user),
         "can_write": has_write_scope(user),
         "pending_invites": pending_invites,