transcripts(workspace): abort stale segment loads, preserve tab in hash, show nav hint
- AbortController is threaded through loadSegmentContent to fetch, prepareScreenFrames,
prefetchThumbnails, and prefetchGroupThumbnails so rapid segment clicks cancel in-flight
work cleanly.
- URL hashes now use #<segment>/<tab> so reloads and shared links preserve tab state;
missing or unknown tabs fall through silently to transcript.
- The [ ] nav hint now becomes visible as soon as buildZoomSegments renders one or more
pills, instead of waiting for the first segment click.
- Added a :focus-visible rule for .tr-tab-pane so the keyboard-focusable pane has a
visible focus ring.