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

ui/snakeboard.go

@@ -45,10 +45,7 @@ func SnakeBoard(sg *snake.SnakeGame) h.H {
 	}
 
 	// Cell size scales with grid dimensions
-	cellSize := 20
-	if state.Width <= 20 {
-		cellSize = 24
-	}
+	cellSize := cellSizeForGrid(state.Width, state.Height)
 
 	var rows []h.H
 	for y := 0; y < state.Height; y++ {
@@ -94,3 +91,22 @@ func SnakeBoard(sg *snake.SnakeGame) h.H {
 	attrs = append(attrs, rows...)
 	return h.Div(attrs...)
 }
+
+func cellSizeForGrid(width, height int) int {
+	maxDim := width
+	if height > maxDim {
+		maxDim = height
+	}
+	switch {
+	case maxDim <= 15:
+		return 28
+	case maxDim <= 20:
+		return 24
+	case maxDim <= 30:
+		return 20
+	case maxDim <= 40:
+		return 16
+	default:
+		return 14
+	}
+}