feat(packaging): add app icon and desktop entry

Scalable SVG app icon (pixel-stream motif, indigo->violet ground) plus
a freedesktop .desktop launcher for the --gui front-end, groundwork for
the first Arch package.

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

assets/pixelpass.desktop

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Type=Application
+Name=pixelpass
+GenericName=Screen Sharing
+Comment=P2P screen sharing over iroh — no port forwarding, no signup
+Exec=pixelpass --gui
+Icon=pixelpass
+Terminal=false
+Categories=Network;RemoteAccess;
+Keywords=screen;share;sharing;remote;p2p;iroh;cast;
+StartupNotify=true

assets/pixelpass.svg

@@ -0,0 +1,16 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="256" height="256" viewBox="0 0 256 256">
+  <title>pixelpass</title>
+  <defs>
+    <linearGradient id="bg" x1="0" y1="0" x2="256" y2="256" gradientUnits="userSpaceOnUse">
+      <stop offset="0" stop-color="#4338ca"/>
+      <stop offset="1" stop-color="#7c3aed"/>
+    </linearGradient>
+  </defs>
+  <rect x="8" y="8" width="240" height="240" rx="56" fill="url(#bg)"/>
+  <!-- pixel stream: squares fading cyan -> white, "passed" toward the arrow -->
+  <rect x="50.25" y="181.29" width="16" height="16" rx="3.2" fill="#2dd5ef"/>
+  <rect x="67" y="124.44" width="20" height="20" rx="4" fill="#62dff2"/>
+  <rect x="96.25" y="82.09" width="24" height="24" rx="4.8" fill="#98e8f6"/>
+  <rect x="134.04" y="55.77" width="28" height="28" rx="5.6" fill="#c9f1f9"/>
+  <path d="M 225.5 57.5 L 183.1 85.8 L 176.9 42.2 Z" fill="#f8fafc"/>
+</svg>