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/ui.h

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