add option to save image location

[cristal-node]

功能描述 / Feature description

download location does exist, but it is for downloading gallery (archive). but when you save single image from inside gallery, it saves under "Pictures" folder.
I propose to save it under "Pictures/fehviewer" or give the option to choose the path. tachiyomi and other manga reader have this option, to have a separate album in gallery.

备注 / Additional details

i'm using android 11, dont know how it is in ios. and #50 addressed the same issue, but it was closed with no response.

自查步骤 / Self-review steps

• 如果您有足够的时间和能力，并愿意为此功能请求提交 PR ，请勾上此复选框 / Pull request is welcome. Check this if you want to start a pull request
• 您已搜索过 Issue Tracker，没有找到类似的请求 / I have searched on Issue Tracker, No duplicate or related open issue has been found
• 您确保这个 Issue 只发起一个功能请求。如果您有多个功能请求，烦请发起多个 Issue / Ensure there is only one feature request in this issue. Please make mutiply issue for mutiply feature request