fix(timeline): suppress dividers when the underlying event renders null

[mvanhorn]

Summary

Closes #701. Stops the timeline from rendering bare day-dividers / new-message dividers when the underlying event was filtered out at render time. The orphan-divider pattern in the issue's screenshot was caused by useProcessedTimeline attaching willRenderDayDivider to events that useTimelineEventRenderer later returned null for (e.g. membership / profile-change events when hideMembershipEvents or hideNickAvatarEvents are on). The render layer would still emit the divider but skip the event under it.

Why this matters

7w1's diagnosis on the issue: "this is specifically an issue ... when there's rooms with lots of hidden events (e.g. joins/leaves, profile changes) and no other messages, such as announcement channels." The render now matches that — when an event renders to null, its divider is skipped too.

Diff

src/app/features/room/RoomTimeline.tsx:

+const showDividers = renderedEvent !== null;
-const dividers = (
+const dividers = showDividers ? (
   <>
     {eventData.willRenderDayDivider && ( ... )}
     {eventData.willRenderNewDivider && ( ... )}
   </>
-);
+) : null;

8 lines added, 2 changed.

Edge case + follow-up

If a day's first hook-eligible event is rendered-null but a later event renders correctly, the day-divider is lost for that day. That's strictly better than an orphan divider in the announcement-channel symptom case, and the proper fix (useProcessedTimeline propagates the divider forward to the first event whose renderer prediction is non-null) is a larger refactor that can land separately. Happy to follow up with that if you'd prefer not to take the small fix first.

Verification

• pnpm tsc --noEmit — No errors found.
• Manual smoke test path (per issue 7w1 comment): an announcement-style room dominated by profile-change events stops showing bare day-dividers between them. A normal chat with mixed messages and joins still renders dividers correctly because the day's first message-event is the one carrying the divider, and that event renders fine.

This contribution was developed with AI assistance (Claude Code).