Replace SD card storage with on-board Flash filesystem (FFat), allocating 9MB internal storage partition for keepsakes

sound_picture_app/Display_PNG.cpp

@@ -7,7 +7,7 @@ static int16_t ypos = 0;
 
 // ── PNG callbacks ─────────────────────────────────────────────────────────────
 static void* pngOpen(const char* filePath, int32_t* size) {
-  Image_file = SD_MMC.open(filePath);
+  Image_file = FFat.open(filePath);
   if (!Image_file) { printf("PNG: failed to open %s\r\n", filePath); return nullptr; }
   *size = Image_file.size();
   return &Image_file;