Fix rounded path corners on angles other than 90 degrees

Corner rounding used a formula that only produces a correct arc at exactly 90 degrees; every shallower or sharper corner came out visibly under-rounded with a kink where the arc met the straight edge. The radius now matches how Figma defines it (the rounding circle’s radius), and the arc math derives the curve from that, so a rounded corner looks like a smooth arc at any angle.