Add blend area effect for layers with distance field mask

Introduces a 'blendArea' property to layers and UI controls for adjusting it. Implements distance field mask generation in ImageAnalysis.ts and applies the mask during layer rendering for smooth edge blending. Refactors CanvasRenderer to delegate layer drawing to CanvasLayers for proper blend area support.

src/types.ts

@@ -20,6 +20,7 @@ export interface Layer {
     mask?: Float32Array;
     flipH?: boolean;
     flipV?: boolean;
+    blendArea?: number;
 }
 
 export interface ComfyNode {