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/persister.go

@@ -172,6 +172,7 @@ func (p *SnakePersister) SaveSnakeGame(sg *snake.SnakeGame) error {
 			GridWidth:  gridWidth,
 			GridHeight: gridHeight,
 			GameMode:   int64(sg.Mode),
+			SnakeSpeed: int64(sg.Speed),
 		})
 		return err
 	}
@@ -224,6 +225,7 @@ func (p *SnakePersister) LoadSnakeGame(id string) (*snake.SnakeGame, error) {
 		Status:  snake.Status(row.Status),
 		Mode:    snake.GameMode(row.GameMode),
 		Score:   int(row.Score),
+		Speed:   int(row.SnakeSpeed),
 	}
 
 	if row.RematchGameID.Valid {