This PR fixes the logic to dismiss the "resolve merge conflict with
agent" button. Previously, we were just observing
`merge_heads_by_conflicted_path`, which seems to be intentionally
sticky, preserving the conflicted paths until changes are either
committed or aborted. This would make the button to resolve conflicts
show up even _after_ the changes get resolved. Now, we're checking
whether paths are _currently_ conflicted (`is_conflicted()`), and if
they are not, we don't display the button, even though the resolution
might have not been committed or aborted yet.

As a bonus, in this PR, I'm also putting the resolve conflict button
_before_ the activity indicator, so as to avoid bounciness, and did a
quick polish of the activity indicator button itself, by using the
`Button` component.

Release Notes:

- Fixed a bug with the merge conflict "resolve with agent" button where
it would be displayed even though all conflicts have already been
resolved.