fix(gui): give the window a real icon instead of the Wayland fallback

Set the Wayland app_id to `pixelpass` so the compositor matches the
installed pixelpass.desktop and uses its Icon= in the titlebar/taskbar,
replacing the generic fallback. Also embed a 256px PNG (rendered from
assets/pixelpass.svg) and set it via with_icon for X11 _NET_WM_ICON, and
add StartupWMClass=pixelpass to the desktop entry for robust window↔entry
matching across desktop environments. No new deps — eframe already pulls
the image crate, and icon_data::from_png_bytes decodes the embed.

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

assets/pixelpass.desktop

@@ -5,6 +5,7 @@ GenericName=Screen Sharing
 Comment=P2P screen sharing over iroh — no port forwarding, no signup
 Exec=pixelpass --gui
 Icon=pixelpass
+StartupWMClass=pixelpass
 Terminal=false
 Categories=Network;RemoteAccess;
 Keywords=screen;share;sharing;remote;p2p;iroh;cast;