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