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.cpp

@@ -31,6 +31,7 @@ void Notifier::update(uint32_t nowMs) {
     if (shown && shown->id != alertedId_) {
         alertedId_ = shown->id;
         alert(shown->level, nowMs);
+        if (onShow) onShow(nowMs, nowMs + kToastMs);
     }
     if (ledOn_ && static_cast<int32_t>(nowMs - ledOffAtMs_) >= 0) {
         rgbLedWrite(kLedPin, 0, 0, 0);