fix: loading state to show loader only if it takes more than 250ms (#7268)

## Summary

To prevent the flash of "loading..." and "calculating dimensions" when
loading cached images only set loading set if longer than 250ms

## Changes

- **What**: ImagePreview.vue
- **Breaking**: <!-- Any breaking changes (if none, remove this line)
-->
- **Dependencies**: <!-- New dependencies (if none, remove this line)
-->

## Review Focus

<!-- Critical design decisions or edge cases that need attention -->

<!-- If this PR fixes an issue, uncomment and update the line below -->
<!-- Fixes #ISSUE_NUMBER -->

## Screenshots (if applicable)

The retrigger loading is because i have throttled 4g slow in the demo.
So cache takes time. Normally this doesn't happen.


https://github.com/user-attachments/assets/335ca7e4-4ce1-43dd-b7d0-9ee88e187069

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7268-fix-loading-state-to-show-loader-only-if-it-takes-more-than-150ms-2c46d73d365081a6b311f78ba3e1cffd)
by [Unito](https://www.unito.io)

tests-ui/tests/renderer/extensions/vueNodes/components/ImagePreview.test.ts

@@ -208,11 +208,6 @@ describe('ImagePreview', () => {
     await navigationDots[1].trigger('click')
     await nextTick()
 
-    // Simulate image load event to clear loading state
-    const component = wrapper.vm as any
-    component.isLoading = false
-    await nextTick()
-
     // Now should show second image
     const imgElement = wrapper.find('img')
     expect(imgElement.exists()).toBe(true)
@@ -265,11 +260,6 @@ describe('ImagePreview', () => {
     await navigationDots[1].trigger('click')
     await nextTick()
 
-    // Simulate image load event to clear loading state
-    const component = wrapper.vm as any
-    component.isLoading = false
-    await nextTick()
-
     // Alt text should update
     const imgElement = wrapper.find('img')
     expect(imgElement.exists()).toBe(true)