Add play again button for rematch after game ends

When a game finishes (win or draw), players see a "Play again" button.
Clicking it creates a new game and the opponent sees a "Join Rematch"
link to join the same game.

game/store.go

@@ -196,6 +196,30 @@ func (gi *GameInstance) GetPlayerColor(pid PlayerID) int {
 	return 0
 }
 
+func (gi *GameInstance) CreateRematch(gs *GameStore) *GameInstance {
+	gi.gameMu.Lock()
+	defer gi.gameMu.Unlock()
+
+	if !gi.game.IsFinished() || gi.game.RematchGameID != nil {
+		return nil
+	}
+
+	newGI := gs.Create()
+	newID := newGI.ID()
+	gi.game.RematchGameID = &newID
+
+	if gi.persister != nil {
+		if err := gi.persister.SaveGame(gi.game); err != nil {
+			gs.Delete(newID)
+			gi.game.RematchGameID = nil
+			return nil
+		}
+	}
+
+	gi.dirty = true
+	return newGI
+}
+
 func (gi *GameInstance) DropPiece(col int, playerColor int) bool {
 	gi.gameMu.Lock()
 	defer gi.gameMu.Unlock()