feat: implement hold piece functionality with swipe up gesture

app/src/main/java/com/mintris/MainActivity.kt

@@ -172,6 +172,15 @@ class MainActivity : AppCompatActivity() {
             binding.nextPieceView.invalidate()
         }
         
+        // Set up hold piece preview
+        binding.holdPieceView.setGameView(gameView)
+        gameBoard.onPieceLock = {
+            binding.holdPieceView.invalidate()
+        }
+        gameBoard.onPieceMove = {
+            binding.holdPieceView.invalidate()
+        }
+        
         // Set up music toggle
         binding.musicToggle.setOnClickListener {
             isMusicEnabled = !isMusicEnabled

app/src/main/java/com/mintris/game/HoldPieceView.kt

@@ -0,0 +1,134 @@
+package com.mintris.game
+
+import android.content.Context
+import android.graphics.BlurMaskFilter
+import android.graphics.Canvas
+import android.graphics.Color
+import android.graphics.Paint
+import android.util.AttributeSet
+import android.view.View
+import com.mintris.model.GameBoard
+import com.mintris.model.Tetromino
+import kotlin.math.min
+
+/**
+ * View that displays the currently held piece
+ */
+class HoldPieceView @JvmOverloads constructor(
+    context: Context,
+    attrs: AttributeSet? = null,
+    defStyleAttr: Int = 0
+) : View(context, attrs, defStyleAttr) {
+
+    private var gameView: GameView? = null
+    private var gameBoard: GameBoard? = null
+
+    // Rendering
+    private val blockPaint = Paint().apply {
+        color = Color.WHITE
+        isAntiAlias = true
+        style = Paint.Style.FILL
+    }
+
+    private val glowPaint = Paint().apply {
+        color = Color.WHITE
+        alpha = 40
+        isAntiAlias = true
+        style = Paint.Style.STROKE
+        strokeWidth = 1.5f
+        maskFilter = BlurMaskFilter(8f, BlurMaskFilter.Blur.OUTER)
+    }
+
+    private val blockGlowPaint = Paint().apply {
+        color = Color.WHITE
+        alpha = 60
+        isAntiAlias = true
+        style = Paint.Style.FILL
+        maskFilter = BlurMaskFilter(12f, BlurMaskFilter.Blur.OUTER)
+    }
+
+    /**
+     * Set the game view reference
+     */
+    fun setGameView(view: GameView) {
+        gameView = view
+        gameBoard = view.getGameBoard()
+    }
+
+    /**
+     * Get the game board reference
+     */
+    private fun getGameBoard(): GameBoard? = gameBoard
+
+    override fun onDraw(canvas: Canvas) {
+        super.onDraw(canvas)
+        
+        // Get the held piece from game board
+        gameBoard?.let {
+            it.getHoldPiece()?.let { piece ->
+                val width = piece.getWidth()
+                val height = piece.getHeight()
+                
+                // Calculate block size for the preview (smaller than main board)
+                val previewBlockSize = min(
+                    canvas.width.toFloat() / (width + 2),
+                    canvas.height.toFloat() / (height + 2)
+                )
+                
+                // Center the piece in the preview area
+                val previewLeft = (canvas.width - width * previewBlockSize) / 2
+                val previewTop = (canvas.height - height * previewBlockSize) / 2
+                
+                // Draw subtle background glow
+                val glowPaint = Paint().apply {
+                    color = Color.WHITE
+                    alpha = 10
+                    maskFilter = BlurMaskFilter(previewBlockSize * 0.5f, BlurMaskFilter.Blur.OUTER)
+                }
+                canvas.drawRect(
+                    previewLeft - previewBlockSize,
+                    previewTop - previewBlockSize,
+                    previewLeft + width * previewBlockSize + previewBlockSize,
+                    previewTop + height * previewBlockSize + previewBlockSize,
+                    glowPaint
+                )
+                
+                // Draw the held piece
+                for (y in 0 until height) {
+                    for (x in 0 until width) {
+                        if (piece.isBlockAt(x, y)) {
+                            val left = previewLeft + x * previewBlockSize
+                            val top = previewTop + y * previewBlockSize
+                            val right = left + previewBlockSize
+                            val bottom = top + previewBlockSize
+                            
+                            // Draw outer glow
+                            blockGlowPaint.color = Color.WHITE
+                            canvas.drawRect(
+                                left - 2f,
+                                top - 2f,
+                                right + 2f,
+                                bottom + 2f,
+                                blockGlowPaint
+                            )
+                            
+                            // Draw block
+                            blockPaint.color = Color.WHITE
+                            canvas.drawRect(left, top, right, bottom, blockPaint)
+                            
+                            // Draw inner glow
+                            glowPaint.color = Color.WHITE
+                            canvas.drawRect(
+                                left + 1f,
+                                top + 1f,
+                                right - 1f,
+                                bottom - 1f,
+                                glowPaint
+                            )
+                        }
+                    }
+                }
+            }
+        }
+    }
+} 

app/src/main/res/drawable/preview_border.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<shape xmlns:android="http://schemas.android.com/apk/res/android"
+    android:shape="rectangle">
+    <stroke
+        android:width="2dp"
+        android:color="#FFFFFF" />
+    <solid android:color="#00000000" />
+</shape>