timeline: fix auto-scroll undershooting on tall messages

[amaanq]

Description

When a tall message (a sticker, a long text block) arrives in an active
room, useLiveEventArrive fires and triggers scrollToBottom(scrollEl, 'smooth') from a layout effect. But reactions and read receipts from
other participants fire the same handler milliseconds later, and each
scrollTo({behavior: 'smooth'}) call cancels the inflight CSS
animation and restarts from wherever the viewport has reached, usually
only a few pixels further. The scroll never converges, leaving the new
message partially off-screen.

This commit replaces the native smooth scroll with a
requestAnimationFrame loop that reads scrollHeight - offsetHeight
each frame and advances scrollTop by 20% of the remaining distance
(~230 ms to 95% at 60 fps). Subsequent live events are no-ops because
the running loop already chases the live bottom. wheel and
touchstart listeners cancel the animation so back-scrolling is
unaffected, and cancelSmoothScroll clears state for instant scrolls
on room switches.

• Fixes Cinny does not scroll to the bottom on receiving a new message. #2259

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

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[amaanq]

I have read the CLA Document and I hereby sign the CLA