Migrate off the OCA.Viewer global before Nextcloud 36

Talk shows file previews that open in the viewer:

• src/components/MessagesList/MessagesGroup/Message/MessagePart/FilePreview.vue:472: OCA.Viewer?.mimetypes?.includes(this.file.mimetype)
• OCA.Viewer.open(...) in the same component
• src/components/.../FilePreview.spec.js stubs OCA.Viewer in several tests

OCA.Viewer.mimetypes.includes(mime) becomes canView(node), which asks the registered
handlers rather than a flattened mime list, so a handler that decides on more than the mime
type is answered correctly too.

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.

[!IMPORTANT]
There is no compatibility shim, which was a deliberate call, so this is a real port rather
than a rename. Nothing breaks until the server PR lands; after that these calls throw on
Nextcloud 36.

[!TIP]
The developer manual covers the migration under
Critical changes
(being added in nextcloud/documentation#15597), and the
README
has the full before/after table. Happy to help with the port, just ping me.

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

Start with src/components/MessagesList/MessagesGroup/Message/MessagePart/FilePreview.vue, especially the references around line 472, then read src/components/.../FilePreview.spec.js and the @nextcloud/viewer migration README. Port the preview checks and opening behavior to the library APIs and update the OCA.Viewer test stubs; done means the relevant tests pass without relying on the global.