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/instance/console_stub.go

@@ -7,3 +7,5 @@ func HideConsole() {}
 func ShowConsole() {}
 
 func ProtectFromConsoleClose() {}
+
+func AllowConsoleKill() {}