Device-width segmented control: three buttons (375 / 768 / 1024) that
drive the iframe's inline width style per 05-UI-SPEC lines 267-280.
The component renders ONLY the toggle buttons — the iframe itself lives
inside MailglassAdmin.Preview.Tabs so the chosen width propagates to
the iframe's inline style.
Accessibility: uses role="group" with aria-label="Preview device width"
and aria-pressed on each button per 05-UI-SPEC Accessibility
Interactions lines 509-514.
Boundary classification: submodule auto-classifies into the
MailglassAdmin root boundary.
Summary
Functions
Renders the 3-button device-width segmented control.