Migrate off the OCA.Viewer global before Nextcloud 36

Text registers a viewer handler and opens the viewer for images:

• src/viewer.js:23: OCA.Viewer.registerHandler({ id: 'text', mimes, component, group, theme, canCompare })
• js/text-viewer.mjs
• src/nodes/ImageView.vue

The handler needs the most attention: handlers are custom elements now, not Vue components
handed to the viewer. You define the element on window.customElements and register its
tagname; enabled(nodes) replaces the mimes array; and canCompare is gone because any
handler can be compared now. The
registration walkthrough
covers the shape.

[!WARNING]
Without this, text and markdown files 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 the registration walkthrough and inspect src/viewer.js:23, js/text-viewer.mjs, and src/nodes/ImageView.vue. Replace the OCA.Viewer integration with @nextcloud/viewer APIs and custom-element handler registration. Done means text and markdown files still open and image viewing works with the new node-based viewer API.