The "Collapse" button doesn't do anything when "Flat" view is selected. Could be disabled in such cases
Originally posted by @m3nu in #1749 (comment)

This is a very minor FR, probably a few lines or so but I wanted to keep it seperate from original PR. I'll submit a PR in a while but please feel free to pick this up if anyone is interested.

Instructions

1. Have a look at

   vorta/src/vorta/views/diff_result.py

   Line 183 in 567a354

   def change_display_mode(self, selection: int):

2. Use setEnabled to disable bCollapseAll in flat mode and enable it in the other modes.
3. Write a test for that behaviour in https://github.com/borgbase/vorta/blob/e85ec38c65f838a1c42dc3d5afdcb3dccb9802f4/tests/unit/test_diff.py