fix(sway/language): apply CSS classes on the main thread to stop SIGSEGV

set_current_layout() mutated label_'s GTK style context (remove_class/
add_class) while being called from the sway IPC worker thread via
onEvent(). Off-main-thread GTK widget mutation caused a SIGSEGV.

Record only the target layout in set_current_layout() and apply the
matching CSS class in update(), which the dispatcher runs on the GTK
main thread. A new applied_class_ member tracks the currently applied
class so update() can swap it. The shared layout_/applied_class_ state
is guarded by the existing mutex_.

Fixes #3702.

include/modules/sway/language.hpp

@@ -54,6 +54,9 @@ class Language : public ALabel, public sigc::trackable {
   const static std::string XKB_ACTIVE_LAYOUT_NAME_KEY;
 
   Layout layout_;
+  // CSS class currently applied to label_. Tracked so update() (main thread) can swap classes
+  // instead of set_current_layout() mutating the widget from the IPC worker thread (#3702).
+  std::string applied_class_;
   std::string tooltip_format_ = "";
   std::map<std::string, Layout> layouts_map_;
   bool hide_single_;