[Enhancement]: Decouple "Global credentials, manually entered" visibility from "Allow people to mount external storage"

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Is your feature request related to a problem? Please describe.
Yes. Currently in Nextcloud Hub 26 Spring (34.0.1), the visibility of the Personal -> External Storages settings page is hardcoded to an administrative toggle found under Administration -> External storage -> Advanced options for external storage mounts called "Allow people to mount external storage".

If an administrator unchecks this box to prevent users from mounting arbitrary or unauthorized external storage backends, Nextcloud completely hides the Personal External Storage UI tab for regular users.

Consequently, this breaks the ability to use the Global credentials, manually entered authentication method for admin-defined shares. If an administrator creates 20+ corporate SMB/CIFS shares centrally and sets them to use "Global credentials, manually entered" (so users only have to type their network password once), users have no way to input or save those credentials. Forcing administrators to use "User entered, store in database" as an alternative creates a terrible user experience, requiring users to authenticate individually for every single one of the 20+ shares.

Describe the solution you'd like
The Personal External Storage tab visibility should be decoupled from user-creation privileges.

When an admin toggles off "Allow people to mount external storage" under "Advanced options for external storage mounts", this should only hide the "Add external storage" option in the user UI, leaving the ability to set global credentials intact.

This layout fix allows administrators to lock down the server from unauthorized local or cloud mounts while still allowing users to maintain a single, centralized credential set for admin-approved network shares.

Describe alternatives you've considered
Using "User entered, store in database" works technically, but forcing users to repeat their username and password 20+ times for 20+ separate network shares creates massive friction and is highly inefficient.

Community discussion link

Start at the Personal → External Storages page and the Administration → External storage → Advanced options for external storage mounts setting described in the issue. Trace how the toggle controls the page and distinguish hiding “Add external storage” from retaining global-credential entry; done means users can save those credentials for admin-defined shares while the add option remains hidden.