feat(design): dialogs open by purpose and keep their commit row pinned (E28 S3f1)

Modal takes a required initialFocus (field / close / action), a field that
loads late takes the focus while it still rests on the close button, and
ModalFooter portals a dialog's commit row under the scrolling body from any
depth. The header and footer fades follow the body's scroll timeline, and the
viewport's interactive-widget=resizes-content keeps the footer above an
on-screen keyboard.

frontend/src/components/OnboardingWizard.escape.dom.test.tsx

@@ -40,7 +40,7 @@ describe("the onboarding wizard's Escape", () => {
       >
         <OnboardingWizard me={me} onClose={onClose} />
         {dialog && (
-          <Modal title="boom" onClose={() => {}}>
+          <Modal initialFocus="close" title="boom" onClose={() => {}}>
             <p>the sync failed</p>
           </Modal>
         )}