fix: prevent duplicate MutationObserver when autoAnimate called twice

[kaigritun]

Problem

When autoAnimate() is called multiple times on the same element (which happens in React StrictMode as components render twice), a new MutationObserver was being created each time. This caused child-added animations to fail because multiple observers were interfering with each other.

Solution

Check if a MutationObserver already exists for the element before creating a new one. This ensures idempotent behavior when autoAnimate() is called repeatedly on the same element.

Changes

-      const mo = new MutationObserver(handleMutations)
-      mo.observe(el, { childList: true })
-      mutationObservers.set(el, mo)
+      if (!mutationObservers.has(el)) {
+        const mo = new MutationObserver(handleMutations)
+        mo.observe(el, { childList: true })
+        mutationObservers.set(el, mo)
+      }

This fix was validated by the issue reporter in #232 who confirmed it resolves the problem.

Fixes #232

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