perf(CustomGameModal): optimize difficulty map rendering with caching

src/components/CustomGameModal.vue

@@ -14,6 +14,7 @@ const fillRate = ref(50);
 const errorMsg = ref('');
 const difficultyCanvas = ref(null);
 const isDragging = ref(false);
+const cachedBackground = ref(null);
 
 const drawMap = () => {
   const canvas = difficultyCanvas.value;
@@ -25,8 +26,11 @@ const drawMap = () => {
   // Clear
   ctx.clearRect(0, 0, width, height);
 
-  // Draw Gradient Background
-  // Optimization: Create an image data once if static, but here it's small enough.
+  // Use cached background if available
+  if (cachedBackground.value) {
+      ctx.putImageData(cachedBackground.value, 0, 0);
+  } else {
+      // Draw Gradient Background (Heavy calculation)
       const imgData = ctx.createImageData(width, height);
       const data = imgData.data;
 
@@ -36,10 +40,6 @@ const drawMap = () => {
       
       for (let y = 0; y < height; y++) {
         for (let x = 0; x < width; x++) {
-      // Map x, y to fillRate, size
-      // y=0 -> size 80 (top), y=height -> size 5 (bottom)
-      // x=0 -> fill 10%, x=width -> fill 90%
-      
           const normalizedX = x / width;
           const normalizedY = 1 - (y / height); // 0 at bottom, 1 at top
           
@@ -48,13 +48,8 @@ const drawMap = () => {
           
           const { value } = calculateDifficulty(fRate, sSize);
           
-      // Color Mapping:
-      // Green (0%) -> Yellow (50%) -> Red (100%)
-      // Hue: 120 -> 0
+          // Color Mapping
           const hue = 120 * (1 - value / 100);
-      
-      // Convert HSL to RGB (Simplified)
-      // Saturation 100%, Lightness 50%
           const [r, g, b] = hslToRgb(hue / 360, 1, 0.5);
           
           const index = (y * width + x) * 4;
@@ -65,6 +60,8 @@ const drawMap = () => {
         }
       }
       ctx.putImageData(imgData, 0, 0);
+      cachedBackground.value = imgData;
+  }
 
   // Draw current position
   // Map current fillRate/size to x,y
@@ -166,6 +163,8 @@ const showAdvanced = ref(false);
 const toggleAdvanced = () => {
     showAdvanced.value = !showAdvanced.value;
     if (showAdvanced.value) {
+        // Reset cache when opening to ensure size is correct if canvas resized
+        cachedBackground.value = null; 
         nextTick(drawMap);
     }
 };