fix: replicate story IDs so peers joining mid-session can key results

[JheisonMB]

Story IDs key every saved result. They were host-local state that never crossed
the wire, which broke in three places at once:

• A peer joining mid-session got undefined. state-sync rebuilds state
  field by field and omitted storyIds/storyId, so every path that indexes
  them threw — renderWaiting, startVoting, loadStories.
• stories-load carried the stories without their IDs. The receiver
  replaced its list and kept the old IDs, leaving the two arrays misaligned:
  results attached to the wrong story.
• storyId was only maintained in startVoting. After nextStory() it
  still pointed at the previous story — on the host too, not just on peers.

The fix removes the field rather than patching each site: storyId is no
longer stored, it is derived from currentIndex when the state is read, so it
cannot drift out of step. storyIds now travels with the stories it
identifies, and incoming lists are rebuilt to the same length as the stories —
an unusable ID is replaced in place, never dropped, since dropping would shift
every later ID onto the wrong story.

Nine tests, including the regression for the undefined state-sync. The reason
this shipped is worth recording: there was not a single test that mentioned
storyId.