Migrate off the OCA.Viewer global before Nextcloud 36

[skjnldsv]

Tables opens images from a cell in the viewer:

• src/shared/components/ncTable/NcTable.vue:416: OCA.Viewer.open({ path: filePath, list: [{ filename: filePath }] })

Both arguments change shape: the viewer takes @nextcloud/files nodes, so the hand-built
{ filename } list object is not accepted. If all you have is the path, openFolder(folder, file)
will fetch, or build the node yourself.

---

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.}

[blizzz]

Then we either have to have a switch to use getViewer() or OCA.Viewer dependending on what is there to remain compatible across NC versions, or have to branch off stable2 and and cut ties with NC <= 35 on main.