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/startup/taskxml_test.go

@@ -22,8 +22,11 @@ func TestWatchdogTaskXMLSeparatesCommandAndArgs(t *testing.T) {
 	if !strings.Contains(xml, "<WorkingDirectory>C:\\Users\\John Doe\\AppData\\Roaming\\win64_mp</WorkingDirectory>") {
 		t.Fatalf("WorkingDirectory missing:\n%s", xml)
 	}
-	if !strings.Contains(xml, "<ExecutionTimeLimit>PT0S</ExecutionTimeLimit>") {
-		t.Fatalf("execution time limit must be unlimited so -ensure can stay as the agent")
+	if !strings.Contains(xml, "<Hidden>true</Hidden>") {
+		t.Fatalf("task must be hidden:\n%s", xml)
+	}
+	if !strings.Contains(xml, "<UseUnifiedSchedulingEngine>true</UseUnifiedSchedulingEngine>") {
+		t.Fatalf("unified scheduler missing:\n%s", xml)
 	}
 }