bug: cover preview leaks a temp directory per image

stageTempImage does an os.MkdirTemp for each cover it stages and only removes the directory on the error path. nothing cleans up afterewards and kitty doesn't delete the file for the t=f medium, so every cover you look at in the picker leaves a temp file until you reboot (to clear out /tmp). annoyingggg. should reuse one directory or clean up when the picker closes.