client/web: fix back button on devices with URL prefix

Move Header component inside Router so that links are relative to the
router base URL.

Updates tailscale/corp#14335

Signed-off-by: Will Norris <will@tailscale.com>

client/web/src/components/app.tsx

@@ -59,8 +59,8 @@ function WebClient({
   ) : (
     // Otherwise render the new web client.
     <>
-      <Header node={data} auth={auth} newSession={newSession} />
       <Router base={data.URLPrefix}>
+        <Header node={data} auth={auth} newSession={newSession} />
         <Switch>
           <Route path="/">
             <HomeView