Pass the current user ID to the media viewer caption renderer
`MediaViewerState` now exposes the current user ID, taken from the room's
session ID in `MediaViewerPresenter`, and `MediaViewerBottomBar` passes it to
`HtmlMessageContent` so formatted captions can display content owned by the
current user differently, matching the timeline renderer.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>