Support alternative volumes for file provider domains

## Feature request: support external/removable volumes for Virtual Files (File Provider) on macOS

Virtual Files sync always stores data under `~/Library/CloudStorage/Nextcloud-` on the boot volume — no way to relocate it. This is
a real gap for large libraries on small/full internal disks, especially since v34 removed the ability to run Classic Sync alongside Virtual
Files on macOS.

**Proposed**: let users choose a volume (APFS + encrypted, per Apple's constraints) for the Virtual Files domain.

**Technical pointer**: macOS 15+ added `NSFileProviderDomain.init(displayName:userInfo:volumeURL:)` and
`NSFileProviderManager.checkDomainsCanBeStoredOnVolume(at:)` for exactly this.

Would it be possible to take this into consideration for future enhancements? My internal disk is starving for space, and my NextCloud installation has some 3 Tb of files (mainly photographs), swapping them in an out is not really feasible. (In the meantime, I am using classic synching), but if FileProvider is the future, this becomes a real limitation for me.

Nextcloud desktop client: 34.0.3 (macOS)

Start by reviewing the macOS File Provider integration around NSFileProviderDomain.init(displayName:userInfo:volumeURL:) and NSFileProviderManager.checkDomainsCanBeStoredOnVolume(at:). Done means users can select a supported APFS encrypted external volume for the Virtual Files domain and the client handles unsupported volumes appropriately.

Written by the indexing model from the issue text.