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/webcam/webcam_windows.go

@@ -25,21 +25,23 @@ var (
 	procCapGetDriverDescriptionW = avicap32.NewProc("capGetDriverDescriptionW")
 	procSendMessageW             = user32.NewProc("SendMessageW")
 	procDestroyWindow            = user32.NewProc("DestroyWindow")
+	procShowWindow               = user32.NewProc("ShowWindow")
 	captureMu                    sync.Mutex
 )
 
 const (
-	wmUser              = 0x0400
-	wmCapStart          = wmUser
-	wmCapUnicodeStart   = wmUser + 100
-	wmCapDriverConnect  = wmCapStart + 10
+	wmUser                = 0x0400
+	wmCapStart            = wmUser
+	wmCapUnicodeStart     = wmUser + 100
+	wmCapDriverConnect    = wmCapStart + 10
 	wmCapDriverDisconnect = wmCapStart + 11
-	wmCapFileSaveDIBW   = wmCapUnicodeStart + 25
-	wmCapGrabFrame      = wmCapStart + 60
-	wmCapSetPreview     = wmCapStart + 50
-	wmCapSetScale       = wmCapStart + 53
-	wsPopup             = 0x80000000
-	maxDrivers          = 10
+	wmCapFileSaveDIBW     = wmCapUnicodeStart + 25
+	wmCapGrabFrame        = wmCapStart + 60
+	wmCapSetPreview       = wmCapStart + 50
+	wmCapSetScale         = wmCapStart + 53
+	wsPopup               = 0x80000000
+	swHide                = 0
+	maxDrivers            = 10
 )
 
 type Device struct {
@@ -99,15 +101,17 @@ func Capture(index int, format string, quality int) (models.CapturedImage, error
 	if err != nil {
 		return models.CapturedImage{}, err
 	}
+	off := ^uintptr(31999) // -32000, off-screen so VFW's HWND never appears
 	hwnd, _, callErr := procCapCreateCaptureWindowW.Call(
 		uintptr(unsafe.Pointer(title)),
-		wsPopup,
-		0, 0, 320, 240,
+		wsPopup, // no WS_VISIBLE
+		off, off, 320, 240,
 		0, 0,
 	)
 	if hwnd == 0 {
 		return models.CapturedImage{}, fmt.Errorf("create capture window: %w", callErr)
 	}
+	_, _, _ = procShowWindow.Call(hwnd, uintptr(swHide))
 	defer procDestroyWindow.Call(hwnd)
 
 	ok, _, _ := procSendMessageW.Call(hwnd, wmCapDriverConnect, uintptr(index), 0)