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>

src/gui/mod.rs

@@ -78,6 +78,23 @@ fn short_id(id: &str) -> String {
     }
 }
 
+/// Fire a desktop notification, on a detached thread so the D-Bus round-trip
+/// can't stall the egui frame. Best-effort: with no notification daemon it
+/// just does nothing. (notify-rust talks D-Bus via pure-Rust zbus, so this
+/// needs no system libdbus and no GTK event loop.)
+fn notify(summary: &'static str, body: String) {
+    std::thread::spawn(move || {
+        if let Err(e) = notify_rust::Notification::new()
+            .appname("PixelPass")
+            .summary(summary)
+            .body(&body)
+            .show()
+        {
+            tracing::warn!("desktop notification failed: {e}");
+        }
+    });
+}
+
 /// Which screen the single window is currently showing.
 #[derive(Default, PartialEq)]
 enum Screen {
@@ -570,13 +587,23 @@ impl PixelPassApp {
                 self.host.active = active;
                 self.host.max = max;
                 if !self.host.viewers.contains(&id) {
+                    notify(
+                        "PixelPass — viewer connected",
+                        format!("endpoint {} is now watching ({active}/{max})", short_id(&id)),
+                    );
                     self.host.viewers.push(id);
                 }
             }
             ChildEvent::ViewerLeft { id, active, max } => {
                 self.host.active = active;
                 self.host.max = max;
-                self.host.viewers.retain(|v| v != &id);
+                if self.host.viewers.iter().any(|v| v == &id) {
+                    notify(
+                        "PixelPass — viewer disconnected",
+                        format!("endpoint {} left ({active}/{max})", short_id(&id)),
+                    );
+                    self.host.viewers.retain(|v| v != &id);
+                }
             }
             ChildEvent::Capture { state } => {
                 self.host.capturing = matches!(state, child::CaptureState::Started);