Add GPIO26 speaker beep and melody test.

Play audio on boot and every 12s via the onboard amp; show status on the LVGL UI and document pins.

Co-authored-by: Cursor <cursoragent@cursor.com>

include/audio_test.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include <Arduino.h>
+
+void audio_init();
+// 短测：三声蜂鸣（开机 / 周期测试）
+void audio_play_beep_sequence();
+// 简短旋律（约 1.5s）
+void audio_play_melody();
+const char *audio_last_status();

include/pins.h

@@ -11,5 +11,8 @@
 #define SD_SCK  18
 #define SD_CS   5
 
+// 板载 SC8002B 功放 → P4 喇叭座（DAC2）
+#define SPEAKER_PIN 26
+
 #define SCREEN_WIDTH  240
 #define SCREEN_HEIGHT 320

include/ui.h

@@ -4,3 +4,4 @@
 
 void ui_create();
 void ui_update(const SdCardInfo &info);
+void ui_set_audio_status(const char *text);