AcceptInviteController attached invited users with a hardcoded member role,
ignoring the invite's role entirely (a viewer invite joined as member). Use the
invite's role on accept, require role on invite creation (no silent default),
and drop the role from the request/CreateInvite defaults. Add the Viewer option
to the member role dropdown (now iterates all roles), hide the dropdown on the
current user's own row, and require the member's email to confirm removal.
Covered by tests asserting the accepted role matches the invited role for
viewer/admin/member, plus invite role validation.
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