The SD panel only knew about uploads; everything the GUEST wrote to the
FAT card (photos, logs) stayed invisible inside the bridge. New
readFat16Image() parses the card image back (BPB-driven, real cluster
chains, VFAT long names, root dir - the inverse of buildFat16Image but
general enough for FatFs-written cards), and the panel gains a live
'Card contents' section reading the bridge's readSdImage() with a
per-file Save (download) button and a Refresh. Round-trip covered in
fat-image.test.ts (16 green).