Light the screen (dimmed) while a Notification's Toast shows

An Off screen turns on dimmed for the Toast's duration so the user can
see what beeped. It isn't user activity: the timeouts aren't restarted,
and a key pressed meanwhile reaches the App.

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

src/ui/notifier.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <functional>
+
 #include "event_bus.h"
 #include "settings.h"
 #include "toast_queue.h"
@@ -14,11 +16,15 @@ class Notifier {
     void update(uint32_t nowMs);  // alerts for a newly shown Toast, ends the LED flash
     ToastQueue& toasts() { return toasts_; }
 
+    // Called when a Toast appears, with the time it will disappear (e.g. to light the screen).
+    std::function<void(uint32_t nowMs, uint32_t untilMs)> onShow;
+
    private:
     void alert(NotificationLevel level, uint32_t nowMs);
 
     const Settings& settings_;
-    ToastQueue toasts_{3000, 8};
+    static constexpr uint32_t kToastMs = 3000;
+    ToastQueue toasts_{kToastMs, 8};
     uint32_t ledOffAtMs_ = 0;
     bool ledOn_ = false;
     uint32_t alertedId_ = 0;