`pick_save` now records the folder of every path the user chooses in a save
or export dialog, per document, and starts that document's next bare-name
dialog (exports, an untitled Save As) there. Precedence: a caller's explicit
directory, then the remembered folder, then beside the document (#1866),
then the shell's default. Save As of a saved document still suggests its own
path. The memory is session-only, keyed by DocId, pruned of closed documents
at the next dialog, and not fed by the web's download-name answers, which
carry no folder.