better grid rendering: avoid thicker lines for adjacent cells

expand each day cell size to overlap the bottom and right borders
with the top and left borders of the next cells

src/ui/calendar.rs

@@ -555,8 +555,8 @@ impl<'a> CalendarView<'a> {
                     let day_bounds = Rectangle {
                         x: cell.x,
                         y: cell.y,
-                        width: cell.width,
-                        height: cell.height,
+                        width: cell.width + 1.0,
+                        height: cell.height + 1.0,
                     };
 
                     // update bounds for the rectangle with the actual days