feat(gui): system tray with opt-in close-to-tray setting

Add a StatusNotifierItem tray (ksni — pure-Rust over the zbus stack
notify-rust already pulls; only new crate is the pastey macro helper).
The icon reflects host/viewer status via its tooltip and offers
Show / Quit; it runs on its own thread, channel-wired to the egui app.

Add a Settings screen with a persisted toggle 'keep running in the tray
when I close the window' (config.toml [gui] close_to_tray), defaulting
OFF so the close button quits as users expect. When ON, closing hides
to the tray on X11 / minimizes on Wayland (which has no protocol to hide
a toplevel) and keeps any live stream running. If no tray is present the
close behaves normally, so the window can never be stranded.

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

Cargo.toml

@@ -37,6 +37,9 @@ eframe = { version = "0.34.2", default-features = false, features = ["glow", "de
 # 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 }
+# System-tray icon (StatusNotifierItem over D-Bus). Pure-Rust, riding the same
+# zbus stack notify-rust already pulls — no GTK, no libappindicator/C libdbus.
+ksni = { version = "0.3", optional = true }
 
 [profile.release]
 lto = "thin"
@@ -46,4 +49,4 @@ 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", "dep:notify-rust"]
+gui = ["dep:eframe", "dep:notify-rust", "dep:ksni"]