luma-gtk: Suggest banner button via CSS class

Adw.Banner.setButton(style:) wraps adw_banner_set_button_style,
a libadwaita 1.6 API — so targeting snap's core24 stack (which
pins libadwaita 1.5) broke the build. Add the suggested-action
class directly so the styling applies on both versions without
requiring the newer accessor.

LumaGtk/Sources/LumaGtk/SessionDetachedBanner.swift

@@ -12,7 +12,7 @@ enum SessionDetachedBanner {
         let banner = title(for: session).withCString { Adw.Banner(title: $0) }
         banner.useMarkup = true
         banner.buttonLabel = "\(session.kind.reestablishLabel)\u{2026}"
-        banner.setButton(style: .suggested)
+        banner.add(cssClass: "suggested-action")
         banner.revealed = true
         banner.sensitive = session.phase != .attaching
         banner.onButtonClicked { _ in