fix(hyprland/workspaces): own debounce timer on main thread, fix UAF

The debounce timer added for flicker prevention was armed from the IPC
listener thread via Glib::signal_timeout().connect(), while its timeout
lambda and the m_updatePending flag ran on the GTK main thread — an
unsynchronized cross-thread data race on GLib timer/main-loop state.
Additionally ~Workspaces() never disconnected the timer, so a pending
timeout could fire on a freed 'this' (use-after-free).

Restore the pre-refactor threading model: onEvent now only mutates state
under m_mutex on the IPC thread and calls dp.emit() (Glib::Dispatcher is
thread-safe). The debounce timer is owned entirely by the main-thread
update() path, which arms/re-arms it on each dispatch and coalesces
bursts into a single refresh. ~Workspaces() disconnects the timer
(guarded) so none outlives the object. Debounce behavior is preserved.

include/modules/hyprland/workspaces.hpp

@@ -227,8 +227,9 @@ class Workspaces : public AModule, public EventHandler {
   sigc::connection m_scrollEventConnection_;
   IPC& m_ipc;
 
+  // Coalesces bursts of Hyprland events into a single UI refresh. Armed and
+  // disconnected only on the GTK main thread (see Workspaces::update).
   sigc::connection m_debounceTimer;
-  bool m_updatePending = false;
 };
 
 }  // namespace waybar::modules::hyprland