Tighten sidebar timeline top inset

[ComputelessComputer]

Reduce the reserved timeline chrome inset and update sidebar timeline expectations.

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Note

Low Risk
CSS-only spacing tweaks in the desktop sidebar timeline; no logic, data, or auth changes.

Overview
Tightens the sidebar timeline top chrome when topChromeInset is on and the Open calendar chip is not reserving extra space.

The default top spacer and sticky bucket header offset move from h-12 / top-12 to h-8 / top-8. The overlay chips (upcoming meeting, Now, etc.) align to top-8 instead of top-12. Layout when the open calendar chip is visible stays h-20 / top-20.

Timeline sidebar tests are updated to assert the new spacing classes.

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