[client/ui] Tray menu opens on click; hide window at startup

Left-click on the tray icon now opens the menu on every platform — the
window is reached through a new "Open NetBird" entry. Only the action
that matches the current daemon state is shown: Connect when
disconnected, Disconnect when connected. The main window starts hidden
and is only surfaced via the tray, single-instance launch, or daemon
events.

client/ui/main.go

@@ -118,7 +118,7 @@ func main() {
 		Title:            "NetBird",
 		Width:            960,
 		Height:           640,
-		Hidden:           false,
+		Hidden:           true,
 		BackgroundColour: application.NewRGB(24, 26, 29),
 		URL:              "/",
 		Mac: application.MacWindow{