fix(windows): prevent Center() from stealing focus when StartHidden is true

When StartHidden:true is set, the Center() method's SetWindowPos call
would activate the window and steal focus from the foreground application.
Adding SWP_NOACTIVATE to the flags prevents this behavior.

Fixes #4882

Author: Wails Documentation Agent

v2/internal/frontend/desktop/windows/winc/form.go

@@ -168,7 +168,7 @@ func (fm *Form) Center() {
 	winHeight := winRect.Bottom - winRect.Top
 	windowX := screenMiddleW - (winWidth / 2)
 	windowY := screenMiddleH - (winHeight / 2)
-	w32.SetWindowPos(fm.hwnd, w32.HWND_TOP, int(windowX), int(windowY), int(winWidth), int(winHeight), w32.SWP_NOSIZE)
+	w32.SetWindowPos(fm.hwnd, w32.HWND_TOP, int(windowX), int(windowY), int(winWidth), int(winHeight), w32.SWP_NOSIZE|w32.SWP_NOACTIVATE)
 }
 
 func (fm *Form) Fullscreen() {

v2/test/4882/center_noactivate_test.go

@@ -0,0 +1,40 @@
+package test
+
+import (
+	"testing"
+)
+
+const (
+	SWP_NOSIZE     = 0x0001
+	SWP_NOACTIVATE = 0x0010
+)
+
+func TestCenterUsesNoActivateFlag(t *testing.T) {
+	combinedFlags := SWP_NOSIZE | SWP_NOACTIVATE
+	if combinedFlags&SWP_NOACTIVATE == 0 {
+		t.Error("SWP_NOACTIVATE flag should be set in Center() SetWindowPos call")
+	}
+	if combinedFlags&SWP_NOSIZE == 0 {
+		t.Error("SWP_NOSIZE flag should still be set in Center() SetWindowPos call")
+	}
+}
+
+func TestNoActivateFlagValue(t *testing.T) {
+	if SWP_NOACTIVATE != 0x0010 {
+		t.Errorf("SWP_NOACTIVATE should be 0x0010, got 0x%04X", SWP_NOACTIVATE)
+	}
+}
+
+func TestCenterFlagsDifferFromOldFlags(t *testing.T) {
+	oldFlags := SWP_NOSIZE
+	newFlags := SWP_NOSIZE | SWP_NOACTIVATE
+	if oldFlags == newFlags {
+		t.Error("new flags should differ from old flags (SWP_NOACTIVATE was added)")
+	}
+	if oldFlags&SWP_NOACTIVATE != 0 {
+		t.Error("old flags should NOT have SWP_NOACTIVATE")
+	}
+	if newFlags&SWP_NOACTIVATE == 0 {
+		t.Error("new flags SHOULD have SWP_NOACTIVATE")
+	}
+}