Collaborator 0.8.2

fix(viewer): scroll to top when opening a different file

The viewer's scroll container persisted its position across file
switches, so opening a new file left the reader scrolled to wherever
they were in the previous one. Reset scrollTop when loadedPath changes.
Keyed on loadedPath so in-place reloads (fs change, focus, wikilink
update) that only refresh content keep the reader's position.