fix: sent messages not appearing + add Lobby/Auth skeleton loaders

- Fix timelineSegments useMemo stale cache: the Perf-5 optimization used
  timeline.linkedTimelines as its only dep, but that reference never changes
  when events are added in-place; adding eventsLength as a dep makes it
  recompute on every new live event so the binary search always finds the
  new item
- Add LobbySkeleton: shimmer placeholder for space lobby (header + hero +
  room list rows) shown while the Lobby chunk lazy-loads
- Add AuthSkeleton: shimmer placeholder for auth pages (logo + server
  picker + form fields) shown while AuthLayout chunk lazy-loads
- Wire both into Router.tsx fallback props

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

src/app/features/room/RoomTimeline.tsx

@@ -557,7 +557,7 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
       base += len;
       return seg;
     });
-  }, [timeline.linkedTimelines]);
+  }, [timeline.linkedTimelines, eventsLength]);
   const liveTimelineLinked =
     timeline.linkedTimelines[timeline.linkedTimelines.length - 1] === getLiveTimeline(room);
   const canPaginateBack =