Dialog and Panel both drew their own bordered/backgrounded card in the
same --bg-elev, so a Panel-based dialog (create-agent, link-matrix-account)
rendered as two concentric cards with a floating close button on the
outer one and no purpose to it.
Give Dialog a "plain" mode that drops its own card chrome (border,
background, padding) and floating close button, and give Panel an
optional onClose that renders a close button at the end of its own
header row instead. AgentsPage's two Panel-backed dialogs now use
plain + Panel's onClose, so the Panel is the dialog's only visible
card. ConfirmDialog (no Panel of its own) is unaffected — plain
defaults to false, unchanged card + floating close button.
Added a ComponentsPage sample demonstrating the plain + onClose
pairing. Verified both dialog modes via a real headless-chromium
screenshot (plain dialog: single card, close button in the header
bar; default dialog: unchanged floating close button).
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