Use relative path for README screenshot

The previous absolute URL pointed at the Gitea instance and was
proxied by GitHub's camo image cache, which kept serving an old
copy after the screenshot was updated. A relative path lets each
forge (GitHub, Gitea) resolve and serve the image from its own
repo blob, bypassing camo.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

README.md

@@ -2,7 +2,7 @@
 
 A small desktop app that picks a random game from your Steam library and lets you launch it directly. Stop staring at your backlog — let the dice decide.
 
-![Steam Dice screenshot](https://gitbutter.xyz/mollusk/steam-dice/raw/branch/main/screenshot.jpg)
+![Steam Dice screenshot](screenshot.jpg)
 
 ## Features