Make RFID failure non-fatal (self-test retries, 4MHz clock, diagnostic log), add beginner Wi-Fi guide + RFID troubleshooting to README

main/wifi_manager.h

@@ -25,6 +25,12 @@ esp_err_t wifi_manager_init(void);
  */
 esp_err_t wifi_manager_start(void);
 
+/**
+ * @brief True if a real SSID is configured (non-empty, not the
+ *        `YOUR_...` placeholder). Station mode will actually run.
+ */
+bool wifi_manager_sta_configured(void);
+
 /**
  * @brief True if the STA has a valid IP.
  */