client/web: cache csrf key when running in CGI mode

Indicate to the web client when it is running in CGI mode, and if it is
then cache the csrf key between requests.

Updates tailscale/corp#13775

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

tsnet/example/web-client/web-client.go

@@ -30,7 +30,10 @@ func main() {
 	}
 
 	// Serve the Tailscale web client.
-	ws, cleanup := web.NewServer(*devMode, lc)
+	ws, cleanup := web.NewServer(web.ServerOpts{
+		DevMode:     *devMode,
+		LocalClient: lc,
+	})
 	defer cleanup()
 	log.Printf("Serving Tailscale web client on http://%s", *addr)
 	if err := http.ListenAndServe(*addr, ws); err != nil {