feat: implement cross-platform hotkey management and update frontend build infrastructure

internal/hotkey/modifier_poll_stub.go

@@ -0,0 +1,10 @@
+//go:build !windows
+
+package hotkey
+
+import xhk "wis-free-v3/internal/xhotkey"
+
+func (l *Listener) modifierPollLoop(required []xhk.Modifier, stop <-chan struct{}) {
+	// Listener.Start rejects modifier-only shortcuts on non-Windows; never called.
+	_, _ = required, stop
+}