Add tests and multi-platform keybind support (Linux, macOS, Windows)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

keybind_darwin.go

@@ -0,0 +1,14 @@
+//go:build darwin
+
+package main
+
+import "golang.design/x/hotkey"
+
+var modMap = map[string]hotkey.Modifier{
+	"ctrl":  hotkey.ModCtrl,
+	"shift": hotkey.ModShift,
+	"alt":   hotkey.ModOption,
+	"cmd":   hotkey.ModCmd,
+	"win":   hotkey.ModCmd,
+	"super": hotkey.ModCmd,
+}