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/include/aes67_cfg.h

@@ -26,6 +26,8 @@ cJSON *cfg_get(const char *group);
 // Firmware-side change of one number (e.g. aes67.session_ver): stored like a POST, no validation.
 // apply = false skips the group's apply callback (e.g. to avoid restarting the stream).
 esp_err_t cfg_set_number(const char *group, const char *key, double value, bool apply);
+// Same for a string value.
+esp_err_t cfg_set_string(const char *group, const char *key, const char *value, bool apply);
 
 // Validation helpers for validate callbacks. Each returns false and fills err on failure.
 bool cfg_check_num(const cJSON *g, const char *key, double min, double max, char *err, size_t n);