add advanced brush cursor visualization

Implemented dynamic brush cursor with visual feedback for size (circle radius), strength (opacity), and hardness (solid/dashed border with gradient). Added overlay canvas system for smooth cursor updates without affecting main rendering performance.

js/Canvas.js

@@ -61,6 +61,15 @@ export class Canvas {
         });
         this.offscreenCanvas = offscreenCanvas;
         this.offscreenCtx = offscreenCtx;
+        // Create overlay canvas for brush cursor and other lightweight overlays
+        const { canvas: overlayCanvas, ctx: overlayCtx } = createCanvas(0, 0, '2d', {
+            alpha: true,
+            willReadFrequently: false
+        });
+        if (!overlayCtx)
+            throw new Error("Could not create overlay canvas context");
+        this.overlayCanvas = overlayCanvas;
+        this.overlayCtx = overlayCtx;
         this.canvasContainer = null;
         this.dataInitialized = false;
         this.pendingDataCheck = null;