fix: Prevent hidden events from disrupting timeline date separators and message grouping

[Yashh56]

Description

This PR fixes an issue where the timeline would sometimes render multiple duplicate Date Separators on the same day. It also fixes a related visual issue where consecutive messages sent by the same user within a minute would fail to collapse (group together) and erroneously repeat the user's avatar.

The root cause was that hidden events (such as m.reaction or edits) were continually updating the prevEvent reference as they were processed. Because they are not rendered on screen, any date boundaries they crossed would leave the dayDivider state stuck as true for the next visible message. Furthermore, their presence between normal messages disrupted the sender matching in the collapsed grouping check.

Changes

• Introduced prevRenderedEvent in RoomTimeline.tsx to strictly track the last visible event rendered in the timeline block.
• Updated the dayDivider and collapsed logic to evaluate against prevRenderedEvent instead of prevEvent.
• prevEvent is still maintained for other underlying logic, such as determining the read-receipt newDivider marker.

Close: #3071