fix(client): keep jumped timeline off live bottom

[Just-Insane]

Description

Follow-up for the reopened jump-button regression in #103.

After the async jump path successfully centered the target event, the room timeline could still think it was pinned to the live bottom from the previous room state. That let the scroll handler immediately chase the bottom again and undo the jump. This change clears the bottom-pinned state before the centered jump scroll and reuses the existing short jump-settle guard so the target event stays in place.

Related repo history checked before opening this PR:

• Charm PR fix(client): stabilize mobile send, jump recovery, and PWA diagnostics #112 fixed the main jump-context loading and event-targeted timeline preservation paths
• Charm issue Jump Buttons Broken #103 was reopened for the remaining post-jump snap-to-bottom behavior

Fixes #103

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

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).
  This PR updates the successful async jump path in RoomTimeline.tsx. When a jumped-to event is found and the timeline scrolls it into view, the code now clears the stale atBottom state first and starts the existing short jump scroll block before calling scrollToIndex(..., { align: 'center' }). That prevents the scroll observer from treating the room as still bottom-pinned and immediately chasing the live bottom after the jump lands. No other timeline-loading logic changes here; this is only a follow-up to keep the already-selected target event stable after the jump succeeds.

[github-code-quality]

Code Coverage Overview

Languages: JavaScript

JavaScript / code-coverage/vitest

The overall coverage remains at 22%, unchanged from the integration branch.

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[github-actions]

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