Admin UI should explain Viewer, Editor, and Owner roles

[hedhoud]

Context
Partition access is role-based, but the UI does not make the practical difference between Viewer, Editor, and Owner obvious at the point where users inspect or grant access.

Problem
Admins and users need to understand what a role allows before assigning it or troubleshooting missing actions. Without inline guidance, role behavior has to be inferred from trial and error.

Expected behavior
Show short role explanations near partition membership and role selection surfaces, so each role clearly maps to its permissions.

[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• Admin UI should show a read-only banner for viewer access #596

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[hedhoud]

Related to #596: this should also cover the viewer document view. When a Viewer opens a partition, the UI should show a short read-only message explaining that they can read documents but cannot upload, delete, or manage the partition.