client/web: add confirmation dialogs

Add confirmation dialogs for disconnecting and stopping advertisement
of a subnet route.

Updates #10261

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

client/web/src/components/app.tsx

@@ -73,6 +73,11 @@ function WebClient({
               currentVersion={node.IPNVersion}
             />
           </FeatureRoute>
+          <Route path="/disconnected">
+            <Card className="mt-8">
+              <EmptyState description="You have been disconnected" />
+            </Card>
+          </Route>
           <Route>
             <Card className="mt-8">
               <EmptyState description="Page not found" />
@@ -129,6 +134,11 @@ function Header({
 }) {
   const [loc] = useLocation()
 
+  if (loc === "/disconnected") {
+    // No header on view presented after logout.
+    return null
+  }
+
   return (
     <>
       <div className="flex flex-wrap gap-4 justify-between items-center mb-9 md:mb-12">