feat(ui): mentionSource on Viewer and HtmlViewer, mention ids on the annotation; ui 0.43.1 (#1574)

A host passes mentionSource once on Viewer (forwarded to both CommentPopover mounts) or HtmlViewer (pinpoint and global composers) instead of rendering the composer itself, and the ids the author kept ride onto the created annotation as the additive Annotation.mentions field, set only when a source was supplied and at least one token survived; the key is absent otherwise, so Plannotator's annotations, drafts, exports, share links, external-annotation endpoints and the feedback archive are unchanged.