Segment demo video by scene with embedded GIF segments (#990)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: thomasnordquist <7721625+thomasnordquist@users.noreply.github.com>

.gitignore

@@ -23,4 +23,16 @@ browser-debug-screenshot.png
 app/.webpack-cache
 
 # Temporary files
-/tmp
+/tmp
+
+# Demo video artifacts
+scenes.json
+segment-*.mp4
+segment-*.gif
+ui-test.mp4
+ui-test.gif
+app.mp4
+app2.mp4
+app720.gif
+qrawvideorgb24.yuv
+intro.png