feat(gui): desktop notification when a viewer joins or leaves

The host screen pops a desktop notification on each viewer join/leave,
so you know someone connected while the window is in the background.

Fired on a detached thread (the D-Bus call never touches the egui
frame) and gated on the same viewer-list transitions, so stopping the
host — which drops the child and stops pumping events — doesn't spray a
notification per remaining viewer.

notify-rust's default features give the pure-Rust zbus backend, so this
adds no system libdbus dependency and no GTK event loop (gui feature
only; the headless build is untouched).

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

Cargo.toml

@@ -34,6 +34,9 @@ ureq = { version = "3", default-features = false, features = ["rustls"] }
 toml = "1"
 chrono = { version = "0.4", default-features = false, features = ["clock", "serde"] }
 eframe = { version = "0.34.2", default-features = false, features = ["glow", "default_fonts", "wayland", "x11"], optional = true }
+# Desktop notifications on viewer join/leave. Default features give the
+# pure-Rust zbus backend (no system libdbus, no image crate).
+notify-rust = { version = "4", optional = true }
 
 [profile.release]
 lto = "thin"
@@ -43,4 +46,4 @@ strip = "symbols"
 [features]
 # Opt-in graphical front-end (pixelpass --gui). Default-off so the headless
 # build never pulls the GUI toolkit tree.
-gui = ["dep:eframe"]
+gui = ["dep:eframe", "dep:notify-rust"]