gui: Spanning Tree page (config + live status)

Add a Spanning Tree page: an on/off toggle driving the existing "stp"
command over /cmd, and a live status section fed by a new /stp.json
endpoint - the elected root bridge (priority + MAC), our path cost,
whether we are the root, and the per-port STP state read live from the
ASIC's MSTP register (same 2-bit encoding stp_setup() writes). Ports are
reported by their physical numbers.

Recovered-from: 3132319, 9365c86

httpd/httpd.c

@@ -690,6 +690,8 @@ void httpd_appcall(void)
 				send_mtu();
 			} else if (is_word(q, "/lag.json")) {
 				send_lag();
+			} else if (is_word(q, "/stp.json")) {
+				send_stp();
 			} else if (is_word(q, "/vlanlist")) {
 				send_vlanlist();
 			} else if (is_word(q, "/config")) {