feat: add video support for intercom lines (WHIP/WHEP, H264) #692

Tracking issue for PR #684, which adds in-call participant video: publish the local camera via WHIP and consume remote video via WHEP (H264/VP8 over SMB), with self-preview, per-tile video grid, source pinning, return-feed and fullscreen controls, plus the video-enabled toggle in production setup.

This is the frontend half of a paired backend+frontend feature. The backend counterpart is Eyevinn/intercom-manager#341 (tracking PR Eyevinn/intercom-manager#315).

Filed to get PR #684 onto board #48 and into review, after it sat unreviewed for two days.