escape ampersand in tray menu labels on Windows

Win32 swallows a lone & in an MFT_STRING menu item as the mnemonic
prefix, so "Help & Support" rendered as "Help  Support". Add a
build-tagged menuLabel() helper that doubles & to && on Windows and is
the identity on macOS/Linux (which render & literally), and apply it to
the About submenu label.

client/ui/tray.go

@@ -459,7 +459,7 @@ func (t *Tray) buildMenu() *application.Menu {
 		SetAccelerator("CmdOrCtrl+,").
 		OnClick(func(*application.Context) { t.svc.WindowManager.OpenSettings("") })
 
-	aboutLabel := t.loc.T("tray.menu.about")
+	aboutLabel := menuLabel(t.loc.T("tray.menu.about"))
 	about := menu.AddSubmenu(aboutLabel)
 	about.Add(t.loc.T("tray.menu.github")).OnClick(func(*application.Context) {
 		_ = t.app.Browser.OpenURL(urlGitHubRepo)

client/ui/tray_label_other.go

@@ -0,0 +1,9 @@
+//go:build !windows && !android && !ios && !freebsd && !js
+
+package main
+
+// menuLabel is the identity on macOS and Linux — both render an ampersand
+// literally in tray-menu labels, so no escaping is needed. Windows opts in via
+// the sibling tray_label_windows.go file, where a lone "&" would otherwise be
+// swallowed as a Win32 mnemonic prefix.
+func menuLabel(s string) string { return s }

client/ui/tray_label_windows.go

@@ -0,0 +1,16 @@
+//go:build windows
+
+package main
+
+import "strings"
+
+// menuLabel escapes a tray-menu label for Win32. A single ampersand in an
+// MFT_STRING menu item is consumed as the mnemonic (accelerator) prefix and
+// never painted — so "Help & Support" renders as "Help  Support". Doubling it
+// to "&&" tells Win32 to draw a literal ampersand. Wails v3 passes the label
+// straight to InsertMenuItem without escaping (see menuitem_windows.go), so we
+// do it here. macOS/Linux render "&" literally and use the identity helper in
+// the sibling tray_label_other.go.
+func menuLabel(s string) string {
+	return strings.ReplaceAll(s, "&", "&&")
+}