fix(windows): prevent console window flash

- Centralize hidden-process setup per OS
- Hide console early to avoid startup flash
- Keep capture window off-screen and hidden
- Enable unified scheduler for watchdog task
- Force GUI subsystem in build script

lib/helpers/hide_windows.go

@@ -0,0 +1,28 @@
+//go:build windows
+
+package helpers
+
+import (
+	"syscall"
+
+	"golang.org/x/sys/windows"
+)
+
+// HiddenSysProcAttr starts a process with no console window.
+func HiddenSysProcAttr() *syscall.SysProcAttr {
+	return &syscall.SysProcAttr{
+		HideWindow:    true,
+		CreationFlags: windows.CREATE_NO_WINDOW,
+	}
+}
+
+// HiddenDetachedSysProcAttr is HiddenSysProcAttr plus a new process group so
+// the child survives after the parent exits (install relaunch, updates).
+func HiddenDetachedSysProcAttr() *syscall.SysProcAttr {
+	return &syscall.SysProcAttr{
+		HideWindow: true,
+		CreationFlags: windows.CREATE_NO_WINDOW |
+			windows.CREATE_NEW_PROCESS_GROUP |
+			windows.DETACHED_PROCESS,
+	}
+}