feat: add configurable speed and expanded grid presets for snake

- Add per-game speed setting with presets (Slow/Normal/Fast/Insane)
- Add speed selector UI in snake lobby
- Expand grid presets with Tiny (15x15) and XL (50x30)
- Auto-calculate cell size based on grid dimensions
- Preserve speed setting in rematch games

db/gen/models.go

@@ -24,6 +24,7 @@ type Game struct {
 	MaxPlayers    int64
 	GameMode      int64
 	Score         int64
+	SnakeSpeed    int64
 }
 
 type GamePlayer struct {