AUGUSTA, Maine — The Maine Senate unanimously endorsed on Wednesday an alternative to a so-called “red flag” bill that was negotiated by Gov. Janet Mills, lawmakers and gun-rights groups, and would peg gun seizures to mental health diagnoses. It’s a bittersweet step for gun-control activists who were hoping to see parts of their agenda adopted at the State House in 2019 under Democratic majorities in the Legislature but were thwarted largely by a Democratic governor wary of prioritizing it. The measure emerged in May as an alternative to a red flag bill that was a top priority of Democrats, including House Speaker Sara Gideon of Freeport.