OTTAWA — The Liberal leadership in the Senate allowed a veteran senator to vote on legislation and spend public dollars for four months after she was diagnosed with dementia and declared legally incompetent. Sen. Joyce Fairbairn regularly attended Senate sittings and voted along party lines before the Upper Chamber rose for the summer at the end of June. Fairbairn was diagnosed with dementia of the Alzheimer’s type by her geriatric psychiatrist in February, according to a letter sent to Senate officials by her niece, Patricia McCullagh. It is unclear when the Liberals knew about the diagnosis, but by April, the...