fix: simplify comment collapse hint for TalkBack

Remove 'Double tap to expand/collapse' instructions. Now only announces
'collapsed' as a state hint when a comment is collapsed and expandable,
and says nothing when expanded.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

improve TalkBack accessibility across stories and comments

- Fix verbose screenReaderLabel: "This story has X points and Y comments"
  is now "X points, Y comments"
- Add full accessibility action set to story tiles: Upvote, Downvote,
  Favorite/Unfavorite, Block/Unblock user, Open in browser, More actions
- Add Upvote and Downvote actions to comment tiles via per-tile VoteCubit
- Label feature discovery tap targets: fav, pin, link, search, jump buttons
- Add barrierLabel: 'Dismiss' to modal bottom sheets to fix 'scrim' announcement
- Hide decorative #N comment index counter from TalkBack with ExcludeSemantics
- Change 'At level X' to 'Level X' in comment body semantics label

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>