Migrate off the OCA.Viewer global before Nextcloud 36 #648

The assistant opens generated media in the viewer:

• src/components/fields/MediaField.vue:315: OCA.Viewer.open({ path: savedPath })
• src/components/fields/ListOfMediaField.vue:338: OCA.Viewer.open({ path: savedPath })

There is no path based entry point any more. Either hand openFolder(folder, file) a folder and
let it fetch, or build the node for the saved file and call open([node], node).

OCA.Viewer is gone in Viewer 7.0.0. The viewer ships as the
@nextcloud/viewer library rather than a
bundled app, and nextcloud/server#63954 removes the viewer app from Nextcloud 36 altogether.

Roughly what it looks like:

import { canView, getViewer } from '@nextcloud/viewer'

// OCA.Viewer.open({ path }) / ({ fileInfo, list })
getViewer().open(nodes, file)          // @nextcloud/files nodes
getViewer().openFolder(folder, file)   // when a path is all you have, let it fetch

// OCA.Viewer.mimetypes.includes(mime) / availableHandlers
canView(node)

The part that is not a rename is that the viewer takes @nextcloud/files nodes now, not
fileinfo objects or path strings. Importing the package is also all it takes to get the viewer
onto the page: no LoadViewer event to dispatch, and nothing to check about whether the app is
enabled.

_{👾 This issue was written with the help of Claude Code.}