Add the Gizmo look for announcements: framed boxes that share a row

gizmolab's "next event" and "next assembly" boxes were hand-written HTML in
the home page's intro; they are announcements by nature, so now they can
be: a fourth style, a box framed in the theme's link colour with an italic
heading. Headed Gizmo boxes in the same spot share a row on wide screens
(the notices wrapper is a flex row, stacking on old browsers like .cols);
one without a heading takes the whole row, which is how a banner line goes
above a pair of boxes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Sd8UPWrvyYCLj97JexNw3A

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