PTP: drop the "Preferred TimeTransmitter" role

It behaved exactly like auto (clock class 248); the only difference was a
priority1 suggestion in the page. Roles are now slave|auto; to prefer this
device, use auto with a low priority1. A stored "master" is converted to
auto at boot (new cfg_set_string, like cfg_set_number).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

components/aes67_web/cfg.c

@@ -295,6 +295,37 @@ esp_err_t cfg_set_number(const char *group, const char *key, double value, bool
     return err;
 }
 
+esp_err_t cfg_set_string(const char *group, const char *key, const char *value, bool apply)
+{
+    if (!s_lock) {
+        return ESP_ERR_INVALID_STATE;
+    }
+    xSemaphoreTake(s_lock, portMAX_DELAY);
+    cfg_group_t *g = find(group);
+    cJSON *cur = g ? cJSON_GetObjectItemCaseSensitive(g->values, key) : NULL;
+    esp_err_t err = ESP_OK;
+    if (!cur || !cJSON_IsString(cur)) {
+        err = ESP_ERR_NOT_FOUND;
+    } else {
+        cJSON_ReplaceItemInObjectCaseSensitive(g->values, key, cJSON_CreateString(value));
+        nvs_handle_t h;
+        if (s_nvs_ok && (err = nvs_open(NVS_NAMESPACE, NVS_READWRITE, &h)) == ESP_OK) {
+            err = store(h, g);
+            if (err == ESP_OK) {
+                err = nvs_commit(h);
+            }
+            nvs_close(h);
+        }
+    }
+    cJSON *copy = err == ESP_OK && apply && g->apply ? cJSON_Duplicate(g->values, true) : NULL;
+    xSemaphoreGive(s_lock);
+    if (copy) {
+        g->apply(copy);
+        cJSON_Delete(copy);
+    }
+    return err;
+}
+
 /* ----- HTTP ----- */
 
 static esp_err_t config_get(httpd_req_t *req)