fix(image): unregisterPreviewUrl never removes an entry from the map

[MILLERMARRU]

Summary

n-image-group's unregisterPreviewUrl had an inverted guard, so it never actually removed an entry from registeredImageUrlMap on unmount.

return function unregisterPreviewUrl() {
  if (!registeredImageUrlMap.value.has(sid)) {
    registeredImageUrlMap.value.delete(sid)
  }
}

Map.prototype.delete() is a no-op when the key is already absent, so this only calls delete() in exactly the case where it does nothing (key already gone), and skips it in the normal unmount case where the key is actually present. So any n-image rendered inside an n-image-group leaves its URL registered in registeredImageUrlMap forever, even after the component unmounts, growing the map with stale entries for images that no longer exist in the DOM.

Fixes #8170

Fix

One-line inversion of the condition:

return function unregisterPreviewUrl() {
  if (registeredImageUrlMap.value.has(sid)) {
    registeredImageUrlMap.value.delete(sid)
  }
}

Scope

This is separate from #8158/#8168 (the registerImageUrl re-registration guard using the wrong key), which is already being fixed in that other PR. This one only touches the unregister path in the same function.

I didn't add a new unit test since registeredImageUrlMap isn't exposed on the component's public instance for a black-box assertion, and imageId is a module-level uuid++ counter (never reused across mounts), so there's no externally-observable symptom to assert on without reaching into internals. Happy to add one if there's a preferred pattern for asserting on this kind of internal state in this codebase.