docs: swap demo poster for user-provided frame

Made-with: Cursor

Author: bschwitz3

README.md

@@ -18,7 +18,7 @@ Tint the iTerm2 background from **Claude Code** session state so many tabs stay
 
 Screen recording (MP4, no audio): **click the image** to open the file in your browser (GitHub “raw” URL — plays inline in most browsers).
 
-[![cctint demo — iTerm2 backgrounds following Claude Code state](./docs/cctint-demo-poster.jpg)](https://github.com/bschwitz3/cctint/raw/main/docs/cctint-demo.mp4)
+[![cctint demo — iTerm2 backgrounds following Claude Code state](./docs/cctint-demo-poster.png)](https://github.com/bschwitz3/cctint/raw/main/docs/cctint-demo.mp4)
 
 Source in repo: [`docs/cctint-demo.mp4`](./docs/cctint-demo.mp4) (H.264 MP4, max width 1280px, no audio track).