Store cross-app cart_sid on login for cart adoption

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bp/auth/routes.py

@@ -49,6 +49,9 @@ def register(url_prefix="/auth"):
     @auth_bp.get("/login/")
     async def login_form():
         store_login_redirect_target()
+        cross_cart_sid = request.args.get("cart_sid")
+        if cross_cart_sid:
+            qsession["cart_sid"] = cross_cart_sid
         if g.get("user"):
             return redirect(federation_url("/"))
         return await render_template("auth/login.html")