Fix Toast timing and alert once per shown Toast

- ToastQueue compared times unsigned: a Toast stamped a few ms after the
  main loop read the clock looked expired, so Toasts were skipped or
  shown late and bursts chained wrongly
- Notifier now beeps and flashes when each Toast appears instead of when
  the Notification arrives, so a burst alerts once per Toast

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

lib/ui/src/toast_queue.h

@@ -11,6 +11,7 @@ namespace roro {
 struct Toast {
     std::string text;
     NotificationLevel level;
+    uint32_t id;  // unique per push, to tell two identical Toasts apart
 };
 
 // Notifications waiting to be shown as Toasts, one at a time, each for the same duration.
@@ -34,6 +35,7 @@ class ToastQueue {
     bool showing_ = false;
     uint32_t shownAtMs_ = 0;
     bool changed_ = false;
+    uint32_t nextId_ = 1;
 };
 
 }  // namespace roro