By Shawn Childs The 49ers are the biggest favorites in Week 18 against the Cardinals at home. San Francisco is favored by two touchdowns. The 49ers have won nine straight games since their 3-4 start, putting them in position to steal the top seed in the NFC if the Eagles trip up in Week 18 versus the Giants. Brock Purdy has a surprising 5-0 record, while gaining 8.0 yards per pass attempt with strength in his completion rate (67.4). San Francisco has the best defense in the league in points allowed (264), leading to 10 opponents scoring 17 points or fewer. The 49ers held 12 teams to fewer than…