feat: Implement dynamic cursor height for double-height characters

src/teletext/renderer.py

@@ -439,8 +439,8 @@ class TeletextCanvas(QWidget):
             if draw_fg and self.cursor_visible and c == self.cursor_x and row == self.cursor_y:
                  painter.setCompositionMode(QPainter.CompositionMode.CompositionMode_Difference)
                  # Difference with white creates inversion
-                 # Note: Cursor follows double height? Probably just the active cell.
-                 painter.fillRect(x, y, self.cell_w, self.cell_h, QColor(255, 255, 255))
+                 h_cursor = self.cell_h * 2 if double_height else self.cell_h
+                 painter.fillRect(x, y, self.cell_w, h_cursor, QColor(255, 255, 255))
                  painter.setCompositionMode(QPainter.CompositionMode.CompositionMode_SourceOver)
         
         return next_occlusion_mask