Rename desktop ID and icon to reverse-DNS scheme

The icon name "steam-dice" triggers freedesktop's compound-name
fallback: when not found, it strips "-dice" and resolves to the
Steam package's icon. Switch to io.github.silvernode.SteamDice
so the fallback can't collide with anything pre-existing.

Also add .gitignore for makepkg build artifacts and pycache.

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

.gitignore

@@ -0,0 +1,10 @@
+# Python
+__pycache__/
+*.py[cod]
+
+# makepkg build artifacts
+pkg/
+src/
+steam-dice/
+*.pkg.tar.zst
+*.pkg.tar.zst.sig