Add desktop integration files and window state persistence

outlay-gtk/data/icons/hicolor/scalable/apps/io.github.outlay.svg

@@ -0,0 +1,21 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 128 128" width="128" height="128">
+  <defs>
+    <linearGradient id="bg" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0%" stop-color="#3584e4"/>
+      <stop offset="100%" stop-color="#1a73e8"/>
+    </linearGradient>
+  </defs>
+  <!-- Background circle -->
+  <circle cx="64" cy="64" r="60" fill="url(#bg)"/>
+  <!-- Wallet body -->
+  <rect x="24" y="38" width="72" height="52" rx="8" fill="#ffffff" opacity="0.95"/>
+  <!-- Wallet flap -->
+  <path d="M24 50 Q24 38 36 38 L88 38 Q96 38 96 46 L96 50 Z" fill="#e8e8e8"/>
+  <!-- Coin circle -->
+  <circle cx="82" cy="64" r="14" fill="#f5c211" stroke="#e0a800" stroke-width="2"/>
+  <!-- Dollar sign on coin -->
+  <text x="82" y="70" font-family="sans-serif" font-size="18" font-weight="bold" fill="#8b6914" text-anchor="middle">$</text>
+  <!-- Arrow out (expense indicator) -->
+  <path d="M40 64 L54 54 L54 58 L66 58 L66 70 L54 70 L54 74 Z" fill="#2ec27e" opacity="0.9"/>
+</svg>