Add system tray with minimize-to-tray (Tartube parity 1.6)

Uses ksni (StatusNotifierItem via zbus) so we keep our pure-Rust /
no-GTK stack — zbus is already pulled in by notify-rust and rfd.

Behavior:
- Tray icon shows the bundled icon.png; left-click brings the window
  back to focus
- Right-click menu: Show / Hide / Quit
- Clicking the window's X button minimizes to tray instead of exiting
- Ctrl+Q (or the tray's Quit item) actually exits

Failure modes are silent and non-blocking: no DBus → app behaves as if
tray flag was off. No StatusNotifier host (e.g. GNOME without the
AppIndicator extension) → icon invisible but app still runs normally,
and the X-button close cancellation only fires when the tray spawn
returned a handle.

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

Cargo.toml

@@ -24,6 +24,7 @@ rand = "0.8"
 # File-picker dialog for the desktop GUI. xdg-portal backend keeps it pure-Rust
 # (zbus, no GTK/libdbus build dep), consistent with notify-rust above.
 rfd = { version = "0.15", default-features = false, features = ["xdg-portal", "tokio"] }
+ksni = { version = "0.3.4", features = ["tokio"] }
 
 [profile.release]
 opt-level = 2