The picker could only choose folders: webkitdirectory switches the OS dialog
into folder mode rather than filtering, so one input cannot offer both. Adds
a drop zone with two browse buttons behind two inputs.
Dropping a folder never worked either — dataTransfer.files does not descend
into directories, so it arrived as one typeless entry and was rejected. Walks
dataTransfer.items instead.
Also collects skipped files into one expandable line rather than a toast each,
which a folder of covers and notes made unusable, and shows the file count and
total size before uploading.
