feat(viewer): drop the bundled viewer app for @nextcloud/viewer

[skjnldsv]

• Resolves: [RFC] merge viewer app into server repository #60480

Summary

The viewer used to be a bundled app that every other app had to check for before
it could register a handler. It is a library now — @nextcloud/viewer, in
nextcloud-libraries/nextcloud-viewer —
and this drops the app.

An app that wants to add a file view imports the package and registers its
handler. It no longer matters whether the viewer app is enabled, or which app
loaded it first: several copies on a page elect the newest between them, and only
that copy fetches the viewer, only when a file is opened.

What is left in the server

apps/files puts a thirteen line init script on every page a file can be opened
from, on the same event the viewer app's own listener used. The script does
nothing but import the package; the handlers for images, video and audio come
with it. dist/files-viewer-init.js is 24 kB, 8.3 kB gzipped.

LoadViewer goes with the app. Three places dispatched it behind a
class_exists() guard to ask the app to load itself; none of them need to now.

enabled_preview_providers becomes a core capability

Which mimes have a preview provider decides what the viewer offers to open — heic,
heif and tiff are viewable through a preview and not otherwise. That was an
initial state the viewer app provided, so anything wanting it had to be on a page
that app had touched. It is core.previews.enabled_providers now, next to the
capabilities clients already read, public share pages included.

"core": {
    "previews": {
        "enabled_providers": ["/image\\/png/", "/image\\/jpeg/", "…"]
    }
}

PreviewManager::getProviders() registers closures and instantiates no provider,
so this costs nothing per request, and it reports nothing when previews are off.

OCA.Viewer is gone, and three callers were still on it

Viewer 7.0.0 dropped the global. These do nothing on current master:

• the versions sidebar — view, compare, and whether to offer either
• the "Open online" button of the open-locally dialog
• the pdf link in the admin settings

They take nodes now. The versions sidebar builds one for the version it shows,
pointing at the version's own dav endpoint rather than at the 250px thumbnail
meant for the list; canView() from the package decides whether to offer the
button, which is the same test the click-to-open action uses.

Third-party apps calling OCA.Viewer.open() — Photos, Talk, Deck, Mail,
Collectives — need to move to the package. There is no shim.

Testing

• tests/playwright/e2e/viewer/, which comes over from the viewer's own
  repository in the first commit: 46 passed, 1 skipped
• frontend unit tests: 198 (vue3) and 818 (legacy), including new specs for
  versionToNode and the capability
• tests/lib/OCS/CoreCapabilitiesTest.php, tests/lib/AppTest.php,
  tests/lib/App/AppManagerTest.php

Upgrades: viewer joins the abandoned apps CleanUpAbandonedApps already
cleans the config rows for.

TODO

• Manual check of the versions compare view and the settings pdf link on a live instance
• Before/after screenshots for the versions sidebar
• Developer documentation: the manual still describes the OCA.Viewer API and the LoadViewer event
• Coordinate with Photos, Talk, Deck, Mail and Collectives before this lands

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[skjnldsv]

Ready for reviews! 💪 🚀