fix(pdf): suppress root view-transition animation to isolate fade to PDF pane only

document.startViewTransition generates both a named pseudo-element for the PDF
container (ol-pdf-viewer) and a root-level pseudo-element that covers the entire
page, causing the editor to fade along with the PDF.

Inject a temporary <style> that sets animation:none on the root pseudo-elements
before starting the transition, then remove it in transition.finished.  Only the
named PDF container crossfades; the editor is unaffected.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/web/frontend/js/features/pdf-preview/util/pdf-js-wrapper.ts

@@ -157,15 +157,23 @@ export default class PDFJSWrapper {
       ) {
         // Chrome 111+ / Edge 111+: document-level View Transition.
         // Give the PDF container a unique view-transition-name so only it
-        // crossfades; the rest of the page stays static.
+        // crossfades.  We also suppress the auto-generated root-level animation
+        // (which would otherwise fade the entire page including the editor) by
+        // injecting a temporary <style> that sets animation:none on the root
+        // pseudo-elements.  The style is removed after the transition finishes.
         const name = 'ol-pdf-viewer'
         this.container.style.viewTransitionName = name
+        const noRootAnim = document.createElement('style')
+        noRootAnim.textContent =
+          '::view-transition-old(root),::view-transition-new(root){animation:none}'
+        document.head.appendChild(noRootAnim)
         const transition = doc_.startViewTransition(async () => {
           setDocument()
           await firstPageRendered
         })
         transition.finished.finally(() => {
           this.container.style.viewTransitionName = ''
+          noRootAnim.remove()
         })
         transition.ready.catch(onTransitionError)
       } else {