A Facebook ad that appeared to offer government help with the cost of a new furnace left Colleen Carter of Courtice, Ontario, with something she wasn't expecting — a 20-year payment plan. For Colleen Carter of Courtice, Ontario, a pitch centered on a rebate became a financing agreement that she said would have followed her until she was 94 — illustrating how misleading home-upgrade advertising can burden people with years of avoidable costs. Her…