Share UI fails with error getting server caps

### Bug description

Clicking the share button on any folder/file in Finder opens the share dialog, which then immediately shows: **"Error getting server caps: Response status code was unacceptable: 401."**

The FileProvider sync extension itself authenticates fine (file sync works normally). The web UI works fine. `curl` with the same credentials against `/ocs/v2.php/cloud/capabilities` returns 200. Only `FileProviderUIExt`'s `kit.getCapabilities()` call gets a 401.

The 401 originates here:
https://github.com/nextcloud/desktop/blob/v33.0.4/shell_integration/MacOSX/NextcloudIntegration/FileProviderUIExt/Sharing/ShareTableViewDataSource.swift#L217-L221

Suspected: the credentials passed via XPC `connection.credentials()` from FileProviderExt → FileProviderUIExt are stale or wrong, since FileProviderExt itself can authenticate to the same server with no issue.

### Steps to reproduce

1. Sign in to Nextcloud desktop client on macOS (webflow auth, plain user/password — no LDAP, no SSO)
2. Right-click any file or folder → Share
3. Share dialog appears, then displays: "Error getting server caps: Response status code was unacceptable: 401."

### Expected behavior

Share dialog loads share list and lets me create a public share link.

### What does NOT fix it

- Quit + relaunch Nextcloud
- Clear FileProvider cache (`fileproviderctl repair`, killing extensions)
- Sign out + sign back in
- Remove account + re-add
- Delete legacy `:0`-suffixed keychain entries
- Update from 33.0.3 → 33.0.4

### Workaround

Share via the web UI.

### Operating system

macOS

### Which version of the operating system you are running.

macOS 15.7.5 (Sequoia)

### Package

Official macOS 12+ universal pkg

### Nextcloud Server version

32.0.5

### Nextcloud Desktop Client version

33.0.4 (also reproduces on 33.0.3)

### Is this bug present after an update or on a fresh install?

Bug appeared on 33.0.3, persists after update to 33.0.4

### Are you using the Nextcloud Server Encryption module?

Encryption is Disabled

### Are you using an external user-backend?

- [x] Default internal user-backend