feat: show no-WFB kiosk fallback only while video absent (#2)

internal/display/fallback.go

@@ -0,0 +1,25 @@
+package display
+
+// FallbackTemplate is the no-video kiosk layout: a cleared black canvas with
+// the fallback lines. While decoded video is present the fallback
+// disappears: the canvas stays plain black so the video window keeps
+// rendering priority without stale text around or behind it, and the exact
+// fallback lines return when video ends.
+type FallbackTemplate struct {
+	fallback StatusTemplate
+	video    *VideoPresence
+}
+
+// NewFallbackTemplate builds a template that draws the given fallback lines
+// only while the shared presence reports no video.
+func NewFallbackTemplate(video *VideoPresence, lines ...string) FallbackTemplate {
+	return FallbackTemplate{fallback: NewStatusTemplate(lines...), video: video}
+}
+
+func (t FallbackTemplate) Draw(canvas Canvas) {
+	canvas.Clear()
+	if !t.video.Active() {
+		drawLines(canvas, t.fallback.lines)
+	}
+	canvas.Flush()
+}