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.svg

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 256 256">
+  <defs>
+    <linearGradient id="bg" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0" stop-color="#2a475e"/>
+      <stop offset="1" stop-color="#1b2838"/>
+    </linearGradient>
+    <linearGradient id="die" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0" stop-color="#e8eef2"/>
+      <stop offset="1" stop-color="#b6c3cc"/>
+    </linearGradient>
+  </defs>
+  <rect width="256" height="256" rx="48" fill="url(#bg)"/>
+  <rect x="52" y="52" width="152" height="152" rx="22" fill="url(#die)" stroke="#8f98a0" stroke-width="2"/>
+  <g fill="#1b2838">
+    <circle cx="90" cy="90" r="13"/>
+    <circle cx="166" cy="90" r="13"/>
+    <circle cx="128" cy="128" r="13"/>
+    <circle cx="90" cy="166" r="13"/>
+    <circle cx="166" cy="166" r="13"/>
+  </g>
+</svg>