Migrate off the OCA.Viewer global before Nextcloud 36

Mail opens attachments in the viewer:

• src/mixins/AttachmentMixin.js:28: OCA.Viewer.mimetypes.includes(fileInfo.mime)
• src/mixins/AttachmentMixin.js:40: OCA.Viewer.open({ ... })

OCA.Viewer.mimetypes.includes(mime) becomes canView(node), and open() takes
@nextcloud/files nodes rather than the fileInfo object.

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/mixins/AttachmentMixin.js at lines 28 and 40, then read the linked nextcloud-viewer migration guidance. Replace the OCA.Viewer usage with the library APIs and convert the file information to @nextcloud/files nodes as needed. Done means attachment viewing works without LoadViewer or viewer-app checks and remains compatible with Nextcloud 36.