fix(files): Adjust ID for skip content buttons

[susnux]

• Resolves: [BITV]: Skip links are not working anymore #41696 for the files app

Summary

As we now use Vue the ID needs to be adjusted.

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)

[susnux]

/backport to stable28

[ShGKme]

Main content may have ids:

1. #app-content-vue in Vue apps from rendered as NcAppContent, usually used in full Vue apps with rendered as NcContent with navigation and sidebar (Talk, Photos, Activity, Users settings, Apps settings) and in Files (there is NcAppContent but no NcContent).
2. Set in Template via id-app-content. By default, it is #app-content for user and #content for public, but can be custom set via id-app-content property. Used by not full Vue apps (not NcContent apps), for example, any page in Personal or Admin settings, Help, Public share.

Skip links always uses the ID from p.2.

This PR fixes the issue for Files app but doesn't fix the issue in general. For example, Photos app, Activity app.

I think we should have a solution that fixes Skip links in a common way, otherwise we need to fix it in each app separately and have no control over it.

[susnux]

think we should have a solution that fixes Skip links in a common way, otherwise we need to fix it in each app separately and have no control over it.

I know this is just the first quick fix. I currently prepare a more general fix on the vue library.

[ShGKme]

A common solution from @nextcloud/vue will work for general apps like Photos, Activity or Talk.

This PR is needed because Files app has more complex mounting with NcAppContent but without NcContent by manual mounting of content, navigation, and sidebar.

[susnux]

/backport to stable28

[backportbot-nextcloud]

The backport to stable28 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable28
git pull origin stable28

# Create the new backport branch
git checkout -b fix/foo-stable28

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts. Resolve them.
git cherry-pick abc123

# Push the cherry pick commit to the remote repository and open a pull request
git push origin fix/foo-stable28

Error: Unknown error

More info at https://docs.nextcloud.com/server/latest/developer_manual/getting_started/development_process.html#manual-backport

[backportbot-nextcloud]

The backport to stable28 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable28
git pull origin stable28

# Create the new backport branch
git checkout -b fix/foo-stable28

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts. Resolve them.
git cherry-pick abc123

# Push the cherry pick commit to the remote repository and open a pull request
git push origin fix/foo-stable28

Error: Unknown error

More info at https://docs.nextcloud.com/server/latest/developer_manual/getting_started/development_process.html#manual-backport