One request per book instead of one for everything, queued in state above the
dialog so closing it no longer cancels the import. A docked tray reports each
book and offers a retry; it clears itself after a clean run, stays put if
anything failed, and holds while hovered.
Also fixes the dialog overflowing on long filenames — Dialog.Content is a grid
and its body needed min-w-0 to shrink below the content's intrinsic width.
