Add /vlanlist HTTP endpoint

Returns a JSON array of all configured VLANs with their IDs and names,
e.g. [{"id":1,"name":""},{"id":20,"name":"IoT"}].

The endpoint iterates VLAN IDs 1..4094 and filters by the validity bit
in sfr_data[0] (0x02), following the same pattern as vlan_create() and
vlan_setup() in rtl837x_port.c.

Also adds a small itoa16_html() helper for emitting decimal numbers
up to 4 digits (analogous to the existing 8-bit itoa_html()), used
for VLAN IDs which can reach 4094. Response builder uses the existing
vlan_name() helper for the name lookup, consistent with send_vlan().

Buffer overflow is prevented by breaking out of the iteration loop at
TCP_OUTBUF_SIZE - 60.

This endpoint is the foundation for upcoming UI improvements
(VLAN selector dropdown and overview table).

httpd/httpd.c

@@ -682,6 +682,8 @@ void httpd_appcall(void)
 				send_mtu();
 			} else if (is_word(q, "/lag.json")) {
 				send_lag();
+			} else if (is_word(q, "/vlanlist")) {
+				send_vlanlist();
 			} else if (is_word(q, "/config")) {
 				send_config();
 			} else if (is_word(q, "/cmd_log")) {