Fix database-access for sharing records

[baronfac]

Issue

When opening CloudSharingView for a record that was not created by the current user, the following error occurs:

PrivateDB can't be used to access another user's zone

This is reproducible in the RemindersApp demo.
The issue is in CloudKitSharing.share(record:configure:), which always accesses privateCloudDatabase (line 178):

let (saveResults, _) = try await container.privateCloudDatabase.modifyRecords(
  saving: [sharedRecord, lastKnownServerRecord],
  deleting: []
)

This fails for records owned by other users, since those exist in the shared database, not the private one.

Fix:
Resolve the database from the record's ID instead:

let (saveResults, _) = try await container.database(for: sharedRecord.recordID).modifyRecords(
  saving: [sharedRecord, lastKnownServerRecord],
  deleting: []
)

This ensures the correct database scope is used regardless of record ownership, and CloudSharingView opens as expected.