Administration: Fix spinner misalignment with 40px buttons

[Anupkankale]

Vertically centers the 20px spinner relative to the 40px button height introduced by the new admin design system. Formula: (40 - 20) / 2 = 10px.

• Base .spinner: margin-top 4px → 10px
• #publishing-action .spinner: remove stale 5px margin-top override
• #addtag .spinner: vertical-align top → middle
• .quick-search-wrap .spinner: margin-top -3px → 10px
• .find-box-search .spinner: add explicit top: 10px for absolute position
• widgets.css: split shared rule, update .remove-inactive-widgets margin

Fixes: https://core.trac.wordpress.org/ticket/65131

Trac ticket:

Use of AI Tools

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