docs(readme): serve the tour as animated webp instead of a 91 MB gif

Same 1110 frames, same 1100x618, same behaviour in the page: it is still an img,
so it starts on its own, loops forever and has no player chrome. GitHub strips
autoplay and loop off a video tag, so a video was never an option here anyway.

91.0 MB to 18.7 MB, and it looks better rather than worse. The gif was capped at
256 colours, which is visible as banding in every gradient the tour passes over;
the webp has none. Encoded at quality 60.

Both files sit in the same release, so the gif stays reachable for anything that
still points at it.

The alt text dropped its duration claim. It said a 60-second tour, and the clip
runs 111 seconds.

README.md

@@ -40,7 +40,7 @@ A self-hosted, real-time collaborative travel planner — with maps, budgets, pa
 
 <div align="center">
 
-<img src="https://github.com/mauriceboe/trek-media/releases/download/readme-assets/TREK1.gif" alt="TREK, 60-second tour" width="100%" />
+<img src="https://github.com/mauriceboe/trek-media/releases/download/readme-assets/TREK1.webp" alt="TREK, a tour of the app" width="100%" />
 
 </div>