fix(keyboard-state): fix segfault on device hotplug

The keyboard-state module crashes with SIGSEGV in libinput_device_ref
when a new input device appears in /dev/input/.

Three bugs fixed:

1. Missing NULL check: tryAddDevice() calls libinput_path_add_device()
   which returns NULL on failure, then immediately passes the result to
   libinput_device_ref() without checking.  On laptops, virtual input
   devices (power buttons, lid switch, etc.) appear and disappear in
   /dev/input/ triggering the hotplug handler; if libinput can't open
   one of these, the NULL return causes the segfault.

2. Missing cleanup on device removal: The IN_DELETE handler erased
   devices from the map without calling libinput_path_remove_device(),
   leaving dangling pointers in the libinput context.

3. Thread safety: libinput_devices_ was accessed from 3 threads
   (main/GTK, libinput_thread_, hotplug_thread_) without any mutex.

Fixes #4851

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

include/modules/keyboard_state.hpp

@@ -3,6 +3,7 @@
 #include <fmt/chrono.h>
 #include <gtkmm/label.h>
 
+#include <mutex>
 #include <set>
 #include <unordered_map>
 
@@ -41,6 +42,7 @@ class KeyboardState : public AModule {
 
   struct libinput* libinput_;
   std::unordered_map<std::string, struct libinput_device*> libinput_devices_;
+  std::mutex devices_mutex_;  // protects libinput_devices_
   std::set<int> binding_keys;
 
   util::SleeperThread libinput_thread_, hotplug_thread_;