Returning nan in tracking task

[juliencegarra]

def is_cursor_in_target(self):
if self.target_radius > 0:
return self.is_cursor_in_radius(self.target_radius)
else:
return float('nan')

float('nan') is truthy in Python, so callers using this value in a boolean context (track.py:78-83) will incorrectly treat "no target defined" as "cursor is in target". This causes wrong cursor color selection.

Consider returning False instead.