mAGI: Add 'Replace' to Knowledge File menu

You can now replace a knowledge file in place. Each knowledge file's menu includes a Replace option that opens the same file picker as Add File and swaps in the new file — keeping the file's position in the list, regenerating its description automatically, and creating no duplicate entries. Dragging and dropping a file with the same name now replaces the existing file automatically instead of returning a "file exists" error. Previously, updating a knowledge file meant deleting it and re-uploading, which lost its description and ordering.