Fill sidebar space in two-person meetings

## Bug

When a meeting is in **sidebar mode** with only two participants, the sidebar area is left empty instead of being occupied by the available participant content.

## Expected behavior

With two participants in sidebar mode, the layout should use the sidebar space rather than leaving it blank.

## Actual behavior

The sidebar space remains empty.

## Acceptance criteria

- In sidebar mode with exactly two participants, no unused empty sidebar area is left.
- The participant layout expands or otherwise occupies the available sidebar space appropriately.

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Requested by **a meeting participant** during a Conclave meeting.

Start by locating the TypeScript frontend code that renders participant layouts in sidebar mode, then reproduce a meeting with exactly two participants. Compare the two-participant layout with other sidebar cases. Done means the available participant content occupies the sidebar space with no unused empty area.

Written by the indexing model from the issue text.