Completes the IconButton extraction by adding `size` and `radius` props and
converting the two remaining outliers that were left out of the first pass:
the desktop header logout button (+layout) and the home search submit button
(+page), both 36px with the medium radius.
IconButton now drives every icon button in the app (except profile/history's,
which the shared-HistoryList change removes separately). Defaults stay 32px /
small radius, so the four already-converted sites are untouched. Rendered
size, radius, and variant match the old markup — no behaviour change, no
version bump.
This also sets up applying the --tap-min touch floor in one place once the
touch-target change lands.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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