Refactor codebase resolving warnings and remove self-package imports

lib/view/pages/timetable/CrossPainter.dart

@@ -0,0 +1,16 @@
+import 'package:flutter/material.dart';
+
+class CrossPainter extends CustomPainter {
+  @override
+  void paint(Canvas canvas, Size size) {
+    final paint = Paint()
+      ..color = Colors.red.withAlpha(200)
+      ..strokeWidth = 2.0;
+
+    canvas.drawLine(const Offset(0, 0), Offset(size.width, size.height), paint);
+    canvas.drawLine(Offset(size.width, 0), Offset(0, size.height), paint);
+  }
+
+  @override
+  bool shouldRepaint(CrossPainter oldDelegate) => false;
+}