Add layer flipH/flipV properties and rendering support

Refactors horizontal and vertical mirroring to toggle flipH/flipV properties on layers instead of modifying image data. Updates rendering logic in CanvasLayers and CanvasRenderer to apply horizontal/vertical flipping via canvas transforms. Adds flipH and flipV to Layer type and includes them in state signature calculation.

src/types.ts

@@ -17,6 +17,8 @@ export interface Layer {
     blendMode: string;
     opacity: number;
     mask?: Float32Array;
+    flipH?: boolean;
+    flipV?: boolean;
 }
 
 export interface ComfyNode {