Let $u$ be the solution of the Dirichlet problem for Laplacian in a Lipschitz domain with boundary data $g$. Then, for every $s\in [1/2,3/2]$,
$$
\| u \|_{H^{s}\,(U)} \leq C \| g \|_{H^{s-1/2}\,\,\,\,(\partial U)} .
$$
This is a classical result of Jerison & Kenig. See the remarks below Theorem 0.5 of that paper.
I believe this estimate and a duality argument implies the result you are looking for.
Edit: Since this answer received a downvote, let me add the details of the little duality argument. If I am making a mistake, I would welcome specific comments pointing out my error.
Let $f \in H^1(\partial \Omega)$ and $h\in L^2 (\partial \Omega)$. Let $u$ and $v$ be their respective harmonic extensions to $\Omega$. Then
\begin{align*}
\bigl| \bigl\langle \Lambda f, h \bigr\rangle \bigr|
=
\biggl| \int_{\Omega} \nabla u \cdot \nabla v \biggr|
& \leq
\| \nabla u \|_{H^{1/2}\,\,(\Omega)} \| \nabla v \|_{H^{-1/2}\,\,\,(\Omega)}
\\ &
\leq
C\| u \|_{H^{3/2}\,\,(\Omega)} \| v \|_{H^{1/2}\,\,(\Omega)}
\\ &
\leq
C\| f \|_{H^1\,(\partial\Omega)} \| h \|_{L^2\,(\partial\Omega)} \,\,,
\end{align*}
where in the last line we used the Jerison-Kenig estimate at both endpoints: $s=1/2$ for $v$ and $s=3/2$ for $u$.
By duality, we obtain the estimate
$$
\| \Lambda f \|_{L^2\,(\partial \Omega)}
\leq
C \| f \|_{H^1\,(\Omega)},
$$
which concludes the argument that $\Lambda$ is continuous from $H^1(\Omega)$ to $L^2(\partial \Omega)$.
I have made this argument for $s=1$, but it is easy to check it works for $s$ precisely in the range $[0,1]$.
You would apply the Jerison-Kenig estimate with $(s+1/2)$ to $u$ and $(-s+3/2)$ to $v$.
