fix(refboard): reference-counted textures prevent shared source crash

When duplicated images share the same assetKey, Assets.unload()
was destroying the shared texture source — crashing all sprites
using it (null addressModeU/alphaMode in WebGL renderer).

TextureManager now tracks refCount per asset. Only calls
Assets.unload() when the last reference is released. ImageSprite
calls release() instead of unload() on texture teardown.

frontend/src/canvas/sprites/ImageSprite.ts

@@ -113,7 +113,7 @@ export class ImageSprite extends Container {
   unloadTexture(): void {
     if (!this.loaded) return;
 
-    this.textures.unload(this.assetKey);
+    this.textures.release(this.assetKey);
 
     // Remove sprite from display tree entirely — avoids PixiJS v8 render crash
     if (this._sprite) {