Fix participant render

[SharonStrats]

Ticket #137

[Copilot]

Pull request overview

Fixes the meeting participants UI flow by correctly selecting an available type index (instead of assuming context.index.private[0] exists), preventing the participants picker from crashing when the private index is missing or not yet loaded.

Changes:

• Normalize webId into an RDF node before loading type indexes.
• Select a usable type index scope (prefer private, otherwise first available) and handle missing indexes with a user-facing error.
• Add error handling for failures when loading type indexes, and a message when the user is not logged in.

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