feat(slint): macOS/Windows system tray (tray-icon) + macOS Dock policy

Add the NSStatusItem (macOS) / Shell_NotifyIcon (Windows) tray via the
tray-icon crate: the same 5-item menu, left-click window toggle, a themed
icon (auto uses a macOS template image that adapts to the menu bar), a
static QBZ tooltip, and live icon-variant switching. tray_icon::TrayIcon is
!Send and (on macOS) must be built on the main thread with NSApplication
running, so it lives in a thread_local created via invoke_from_event_loop;
its events are drained from a slint::Timer.

macOS close-to-tray reuses the cross-platform Slint hide/show path. The
opt-in Hide Dock icon toggle switches the NSApplication activation policy to
.accessory while closed to the menu bar (.regular by default, Spotify-style)
via objc2-app-kit.

cc

crates/qbz-slint/Cargo.toml

@@ -67,5 +67,24 @@ qbz-credentials = { path = "../qbz-credentials" }
 [target.'cfg(target_os = "linux")'.dependencies]
 ksni = { version = "0.3", features = ["blocking"] }
 
+# macOS (NSStatusItem) + Windows (Shell_NotifyIcon) tray via the tao team's
+# tray-icon crate. Re-exports muda as tray_icon::menu. Windows is built but
+# not shipped/tested.
+[target.'cfg(any(target_os = "macos", target_os = "windows"))'.dependencies]
+tray-icon = "0.24"
+
+# macOS Dock-icon activation-policy toggle. Pinned to tray-icon's objc2 0.6 /
+# app-kit 0.3 series (NOT winit's 0.5) so MainThreadMarker/NSApplication unify
+# with tray-icon. setActivationPolicy is gated behind the NSRunningApplication
+# feature.
+[target.'cfg(target_os = "macos")'.dependencies]
+objc2 = "0.6"
+objc2-app-kit = { version = "0.3", default-features = false, features = [
+    "std",
+    "NSApplication",
+    "NSResponder",
+    "NSRunningApplication",
+] }
+
 [build-dependencies]
 slint-build = "1.16"