asyncBtn() (shared/src/forms.js) is used by both the dashboard and the
per-agent UI, but its .spinner class + @keyframes spin animation only
lived in dashboard.css. The agent UI's loading spinner rendered as a
static unstyled glyph instead of the animated amber spinner the
dashboard gets. Moved the rule to shared/base.css, which both
common.css (dashboard) and agent.css already @import.
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