Add desktop integration and Arch PKGBUILD

- steam-dice.svg: flat dice icon on Steam-blue gradient
- steam-dice.desktop: launcher entry under Game category
- PKGBUILD: steam-dice-git, builds from GitHub source
- Use setDesktopFileName for correct Wayland WM_CLASS, with
  local SVG fallback when running from the source tree

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

steam-dice.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=steam-dice
+Terminal=false
+Categories=Game;
+Keywords=steam;dice;random;game;launcher;
+StartupNotify=true
+StartupWMClass=steam-dice