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>

io.github.silvernode.SteamDice.desktop

@@ -0,0 +1,12 @@
+[Desktop Entry]
+Type=Application
+Name=Steam Dice
+GenericName=Random Game Picker
+Comment=Pick a random game from your Steam library
+Exec=steam-dice
+Icon=io.github.silvernode.SteamDice
+Terminal=false
+Categories=Game;
+Keywords=steam;dice;random;game;launcher;
+StartupNotify=true
+StartupWMClass=io.github.silvernode.SteamDice