feat(tools): remove background from animated GIFs (WebP, APNG) (#502)

Adds a dedicated remove-gif-background AI tool: removes the background from an animated GIF, WebP, or APNG frame by frame and reassembles a transparent (or composited) animation in WebP, APNG, or GIF, with full per-frame effects. Reuses the background-removal bundle. Verified end-to-end with the real rembg model.

Closes #496.

tests/fixtures/index.ts

@@ -62,6 +62,7 @@ export const fixtures = {
     animated: {
       gif: p("image/valid/animated.gif"),
       webp: p("image/valid/animated.webp"),
+      apng: p("image/valid/animated.apng"),
       real: p("image/valid/animated-simpsons.gif"),
     },
     ocr: {