SOLIDPOD: prevent external file recipient from changing file owner's access permissions

Describe the Bug

Solidpod should sensibly handle the situation when the recipient of an external file tries to change the permissions of the owner of the file. In notepod app, using the GrantPermissionUI, a recipient of an external note can select the note owner as the individual recipient and assign new permissions and clicks 'Grant Permission' button which causes a granting permission error shown as snackbar message:

 'Permission granting failed. Check console logs...' 

Console message:

flutter: 💥 [GrantPermission] Exception occurred: 'package:solidpod/src/solid/utils/permission.dart': Failed assertion: line 68 pos 12: '!thirdPartyAccess.containsKey(ownerWebId)': is not true.

To Reproduce

Steps to reproduce the behavior in Notepod:

1. Go to 'Shared Notes'
2. Click on any note.
3. Click share button
4. On clicking 'Individual' to share to an individual, the suggestion list naturally shows webid's of people with access to the note, excluding the user.

Closing Crieria

• App provides a useful message, e.g. that a note recipient is unable to change the access of a note owner of an external file
• App does not hang

Screenshots

Here I am logged in as jesscmoore and have clicked Grant Permissions to edit permissions of the file owner jessicamoore.

Context

Where has the issue been observed:

• iOS
• Web

App Version: 0.2.23
Flutter Version: Flutter 3.32.8