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>

src/common/config.rs

@@ -1,8 +1,7 @@
 //! Persistent user-level config at `~/.config/pixelpass/config.toml`.
 //!
-//! Right now this only tracks the bandwidth pre-flight result. Future
-//! preferences (default player, default bitrate, etc.) can hang off the
-//! same file under their own `[section]`.
+//! It tracks the bandwidth pre-flight result and the GUI's preferences.
+//! Further settings can hang off the same file under their own `[section]`.
 
 use anyhow::{Context, Result};
 use chrono::{DateTime, Utc};
@@ -16,6 +15,18 @@ use std::path::PathBuf;
 pub struct Config {
     #[serde(default)]
     pub bandwidth: BandwidthEntry,
+    #[serde(default)]
+    pub gui: GuiSettings,
+}
+
+/// Preferences for the `pixelpass --gui` front-end.
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+pub struct GuiSettings {
+    /// When true, the window's close button hides the app to the system tray
+    /// (keeping any live stream running) instead of quitting. Defaults to
+    /// false — closing quits, which is what people expect.
+    #[serde(default)]
+    pub close_to_tray: bool,
 }
 
 /// Result of the first-run upstream measurement.