feat(viewer): keep the user's viewer settings

[skjnldsv]

• Resolves: Adjust the speed (delay between images) of the autoplay feature viewer#447

Summary

The viewer library now has a settings dialog (nextcloud-libraries/nextcloud-viewer#136), and remembers the volume of videos and audio. These settings are stored on the server.

Following review, apps/viewer is a minimal, always enabled app again, the server side of the viewer. It already held the OCA.Viewer shim and the deprecated LoadViewer event. It now also keeps the user's viewer settings, the way the Files app keeps its own: a UserConfig service listing the allowed keys, an OCS endpoint to change one, and the settings handed to the viewer as the viewer/config initial state, for logged-in users only. They are the slideshow delay (1 to 60 seconds), the volume (0 to 100) and whether media start muted.

Core keeps loading the viewer itself. The app is back in shipped.json where it was before #63954, and off the abandoned apps the repair step cleans up. It continues the old app's versions at 9.0.0-dev.0, where nextcloud/viewer's master stood after 8.0.0 in 35, so an instance upgrading from 35 updates it like any other app.

curl -u alice:alice -X PUT -H 'OCS-APIRequest: true' -H 'Content-Type: application/json' \
  https://cloud.example/ocs/v2.php/apps/viewer/api/v1/config/volume \
  -d '{"value": 40}'

The endpoint takes JSON: value is a number or a boolean depending on the setting, and the framework doesn't cast a union from form data.

Checked on a fresh install and on one with the old viewer's installed_version 8.0.0, which occ upgrade updates to 9.0.0-dev.0.

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

👾 This pull request was assisted by Claude Code, commits carry an Assisted-by trailer.

[skjnldsv]

Really not sure about this. Else we have to create an app folder again for viewer

[susnux]

I am not sure its a good thing we move more of viewer into lib.
Maybe rollback and make apps/viewer again a force enabled app to separate concerns here making it cleaner on where to put this.

[skjnldsv]

I am not sure its a good thing we move more of viewer into lib. Maybe rollback and make apps/viewer again a force enabled app to separate concerns here making it cleaner on where to put this.

Maybe a minimal viewer app is necessary indeed. But this is also frustrating, there is no clean way to have libs register user settings without some php scafolding 😤😤

[skjnldsv]

@susnux, alright, done! I didn't use the provisioning api event-based approach in the end, I don't know, don't like it 🙈
I used the good old controller approach, like we do in Files.