Fix when agent-powered merge conflict button shows up

[danilo-leal]

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.

[ConradIrwin]

Fabulous, thank you!

[aviatesk]

Looks like the activity indicator polish in this PR introduced a regression: the spinner no longer animates. content_to_render still produces Content::icon values with .with_rotate_animation(...) for several cases (LSP $/progress, debug session startup, long-running git/fs ops), but the new render path only checks content.icon.is_some() and then always renders a static IconName::LoadCircle:

.when(content.icon.is_some(), |this| {
    this.start_icon(
        Icon::new(IconName::LoadCircle)
            .color(Color::Muted)
            .size(IconSize::Small),
    )
})

The previous ButtonLike-based code rendered content.icon directly via .children(content.icon), preserving both the icon kind (Warning / Download / ArrowCircle) and the rotate animation. After this PR everything collapses to a non-animated LoadCircle, so LSP progress no longer spins in the status bar.