Migrate off the OCA.Viewer global before Nextcloud 36

Notes dispatches the viewer's load event:

• lib/Controller/PageController.php:18: use OCA\Viewer\Event\LoadViewer;
• lib/Controller/PageController.php:71: if (\OCP\Server::get(IAppManager::class)->isEnabledForUser('viewer') && class_exists(LoadViewer::class))
• lib/Controller/PageController.php:72: $this->eventDispatcher->dispatchTyped(new LoadViewer())

[!NOTE]
This one is not urgent. The class_exists() guard means the dispatch is skipped rather than
fatal once the class is gone, so nothing breaks. It just becomes dead code, along with the
isEnabledForUser('viewer') check, since there is no viewer app to enable and importing
@nextcloud/viewer is what puts the viewer on a page now.

Notes does not call OCA.Viewer from javascript, so there is nothing else to port here.

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 in lib/Controller/PageController.php at the import and the LoadViewer dispatch around lines 18 and 71-72. Confirm that Notes has no JavaScript OCA.Viewer calls, then remove the obsolete viewer integration and verify the project checks or tests pass; done means no remaining LoadViewer or viewer-app enablement code is needed here.