fix(mdns+webui): pass NULL instance name + return hub_name in /api/v1/info

Two bench-discovered bugs in v6.2.0-alpha.2 that made AmbiSense show
up in HA's SmartGhar integration with broken labels:

netmgr.c — mDNS instance name leak:
  mdns_service_add("_smartghar", "_smartghar", "_tcp", ...) passed the
  raw service label as the friendly instance name (1st arg). Result:
  HA's discovery card showed "_smartghar [random]" instead of the
  device's name. Fix: pass NULL so the service inherits the global
  mdns_instance_name_set("AmbiSense") set above.

webui.c — missing hub_name field:
  smartghar-homeassistant's SmartGharHubSensor.device_info reads
  info.get("hub_name") to populate HA's device-registry name.
  Without it, the integration falls back to "SmartGhar Hub
  (XXXXXX)" which is ugly. TankSync emits hub_name as the hostname
  ("tanksync-f6dc"); AmbiSense now does the same ("ambisense-f6dc").

Bench-tested: discovery card now reads "AmbiSense" instead of
"_smartghar"; device-registry name populates as "ambisense-XXXX"
matching the host. Combined with the existing product=ambisense
field, the device-registry model resolves correctly to "AmbiSense
Hub" via hub_model_for_product() in smartghar-homeassistant v0.7.0.

Author: Ravi Singh

firmware/components/netmgr/netmgr.c

@@ -154,7 +154,12 @@ static esp_err_t bring_up_mdns(void) {
     char hub_id[40];
     snprintf(hub_id, sizeof(hub_id), "ambisense_%02x%02x%02x%02x%02x%02x",
              mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);
-    if (mdns_service_add("_smartghar", "_smartghar", "_tcp", 80, NULL, 0) == ESP_OK) {
+    /* First arg = friendly instance name. Pass NULL so the service
+     * inherits `mdns_instance_name_set("AmbiSense")` above; passing
+     * "_smartghar" (matching the service type) leaks the raw service
+     * label into HA's discovery card as "_smartghar [random]" instead
+     * of the device's name — which is the bug we hit on 2026-05-07. */
+    if (mdns_service_add(NULL, "_smartghar", "_tcp", 80, NULL, 0) == ESP_OK) {
         mdns_txt_item_t txt[] = {
             { "hub_id",       hub_id        },
             { "product",      "ambisense"   },

firmware/components/webui/webui.c

@@ -901,6 +901,13 @@ static esp_err_t handle_v1_info(httpd_req_t *req) {
     cJSON_AddStringToObject(r, "unique_id", hub_id);  /* alias for older integration code */
     char host[33] = {0}; netmgr_get_hostname(host, sizeof(host));
     cJSON_AddStringToObject(r, "host", host);
+    /* hub_name — what the smartghar HA integration uses as the device
+     * registry "name" (see custom_components/smartghar/sensor.py
+     * SmartGharHubSensor.device_info). Without this, the integration
+     * falls back to "SmartGhar Hub (XXXXXX)" which is ugly. We use
+     * the same hostname-as-friendly-name convention TankSync does
+     * (e.g. "ambisense-f6dc"). */
+    cJSON_AddStringToObject(r, "hub_name", host);
     char ip[24] = {0}; netmgr_get_ip(ip, sizeof(ip));
     cJSON_AddStringToObject(r, "ip", ip);
     cJSON_AddNumberToObject(r, "uptime_s",  (uint32_t)(esp_timer_get_time() / 1000000));