Keep sidebar update control fixed

[ComputelessComputer]

Move the update button into the sidebar control cluster and cover the placement with a regression test.

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Note

Low Risk
Desktop sidebar layout and interaction tests only; no auth, data, or backend changes.

Overview
Moves expanded timeline sidebar controls (toggle, search, new note, devtools, update) out of the absolute data-left-sidebar-chrome overlay and into the resizable sidebar via a timelineHeader slot on ClassicMainSidebar / LeftSidebar (data-sidebar-timeline-header).

The desktop update action now sits in the same button group as the other sidebar controls instead of on the far end of the chrome row. Collapsed sidebar still uses the floating chrome overlay; wheel-to-scroll on the timeline is wired on the new header (onWheelCapture) rather than the old overlay.

TimelineView only uses topChromeInset when there is no injected header, so the timeline does not double-reserve top space. Tests were updated for the new DOM and to lock update-button placement.

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[netlify]

✅ Deploy Preview for old-char canceled.

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[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 2 potential issues.

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