Frontend: Roles — Prevent joining program lines as speaker without permission

[bjowe]

Part of the Roles feature — see parent issue #469.

Program line speaker mode should be restricted to users with the appropriate role. Users without permission should be able to listen but not speak on program lines.

Acceptance criteria:

• Speaker join button disabled or hidden for users without speaker role
• Listener mode still available to all users on program lines
• Error shown if an unpermitted join is attempted

Sub-issue of #469