appview: repo/fragments: swap expand/collapse icon

I noticed that the "Expand all" button uses the icon where the arrows point to the middle and the "Collapse all" button uses the icon where the arrows point outwards consistently across the tangled UI. So I decided to swap the icons. I think the file where I changed it is the only place this is. It's my first PR with jj, sorry if anything is missing or something. If we need another round, at least I can train doing the commit adaptation with jj 😅 Signed-off-by: Felix Schneider <did:plc:pbjvqaziagcyv2vqodldn5op>