Migrate off the OCA.Viewer global before Nextcloud 36

files_mindmap registers a handler and drives the viewer on public shares:

• src/mindmapviewer.js:18: OCA.Viewer.registerHandler({ ... })
• src/mindmap.js:206: OCA.Viewer.openWith('mindmap', { path: node.path })
• src/public.js:22: OCA.Viewer.setRootElement('#files-public-content')
• src/public.js:23: OCA.Viewer.open({ path: '/' })

Handlers are custom elements now, not Vue components handed to the viewer. You define the
element on window.customElements and register its tagname, and enabled(nodes) replaces the
mimes array. See the
registration walkthrough.
openWith('mindmap', { … }) becomes getViewer().open(nodes, file, options, 'mindmap').

[!NOTE]
setRootElement() is gone with no replacement. The viewer mounts its own container on the
body and there is no way to ask it to mount elsewhere. If mounting inside
#files-public-content is load bearing for the public share layout rather than a leftover,
say so on this issue and we will work out what the library should offer.

[!WARNING]
Without this, mindmaps stop opening in the viewer entirely.

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/mindmapviewer.js, src/mindmap.js, and src/public.js, then read the linked @nextcloud/viewer registration walkthrough and migration table. Replace the OCA.Viewer integration with custom-element registration and the new viewer APIs, handling @nextcloud/files nodes. Done means mindmaps still open from the editor and public shares on Viewer 7.0.0 without the removed global.