fix: actualizar la forma de aplicar opacidad en colores de tema y componentes de la interfaz, y se arreglaron errores de interfaz

lib/core/theme/app_theme.dart

@@ -55,7 +55,7 @@ abstract final class AppTheme {
             borderRadius: BorderRadius.circular(12),
           ),
           elevation: 3,
-          shadowColor: leafGreen.withOpacity(0.4),
+          shadowColor: leafGreen.withValues(alpha: 0.4),
           textStyle: const TextStyle(
             fontSize: 16,
             fontWeight: FontWeight.w700,
@@ -89,12 +89,12 @@ abstract final class AppTheme {
           borderSide: const BorderSide(color: errorRed, width: 2),
         ),
         labelStyle: const TextStyle(color: midGray),
-        hintStyle: TextStyle(color: midGray.withOpacity(0.7)),
+        hintStyle: TextStyle(color: midGray.withValues(alpha: 0.7)),
       ),
       cardTheme: CardThemeData(
         color: Colors.white,
         elevation: 2,
-        shadowColor: Colors.black.withOpacity(0.08),
+        shadowColor: Colors.black.withValues(alpha: 0.08),
         shape: RoundedRectangleBorder(
           borderRadius: BorderRadius.circular(16),
         ),