ipn/ipnlocal: serve web client on quad100 if enabled

if the user pref and nodecap for the new web client are enabled, serve
the client over requests to 100.100.100.100.  Today, that is just a
static page that lists the local Tailcale IP addresses.

For now, this will render the readonly full management client, with an
"access" button that sends the user through check mode.  After
completing check mode, they will still be in the read-only view, since
they are not accessing the client over Tailscale.

Instead, quad100 should serve the lobby client that has a "manage"
button that will open the management client on the Tailscale IP (and
trigger check mode). That is something we'll fix in a subsequent PR in
the web client code itself.

Updates tailscale/corp#14335

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

ipn/ipnlocal/local.go

@@ -3113,6 +3113,9 @@ var (
 // apply to the socket before calling the handler.
 func (b *LocalBackend) TCPHandlerForDst(src, dst netip.AddrPort) (handler func(c net.Conn) error, opts []tcpip.SettableSocketOption) {
 	if dst.Port() == 80 && (dst.Addr() == magicDNSIP || dst.Addr() == magicDNSIPv6) {
+		if b.ShouldRunWebClient() {
+			return b.handleWebClientConn, opts
+		}
 		return b.HandleQuad100Port80Conn, opts
 	}
 	if !b.isLocalIP(dst.Addr()) {