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.cpp

@@ -9,7 +9,7 @@ void ToastQueue::push(const std::string& text, NotificationLevel level, uint32_t
         queue_.erase(queue_.begin() + (showing_ && queue_.size() > 1 ? 1 : 0));
         if (!showing_) changed_ = true;
     }
-    queue_.push_back({text, level});
+    queue_.push_back({text, level, nextId_++});
     if (!showing_) {
         showing_ = true;
         shownAtMs_ = nowMs;
@@ -19,7 +19,9 @@ void ToastQueue::push(const std::string& text, NotificationLevel level, uint32_t
 
 const Toast* ToastQueue::current(uint32_t nowMs) {
     if (!showing_) return nullptr;
-    if (nowMs - shownAtMs_ >= durationMs_) dismiss(shownAtMs_ + durationMs_);
+    // Signed: a caller's clock reading may be a few ms older than the Toast's stamp.
+    int32_t elapsed = static_cast<int32_t>(nowMs - shownAtMs_);
+    if (elapsed >= static_cast<int32_t>(durationMs_)) dismiss(shownAtMs_ + durationMs_);
     return showing_ ? &queue_.front() : nullptr;
 }