Tab row, base switcher and document page: small geometry tweaks

[WaylandYang]

A batch of header and library tweaks, reviewed on the dev server before merging.

• Tab row. Keeps its 16 px inset (first tab's icon at 32 with the wordmark); each tab gains 4 px on its right (padding 16 left, 20 right) so the row breathes without moving. The header's gap goes to 16 so the base switcher's icon lands on the second tab's icon (132).
• Base switcher. Side padding 24 → 16, on the pill and the panel rows alike.
• Document page. The sections and their extracted-facts column sit left at the library list's inset (px-8 py-6) instead of centred, and the width cap rises from 4xl to 6xl, so the section column gets about 820 px instead of 580.

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