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/ui/Modal.strictmode.dom.test.tsx

@@ -43,7 +43,7 @@ describe("Modal focus under StrictMode", () => {
   it("keeps the focus its autoFocus child asked for across the double mount", () => {
     render(
       <StrictMode>
-        <Modal title="t" onClose={() => {}}>
+        <Modal initialFocus="close" title="t" onClose={() => {}}>
           <input aria-label="field" autoFocus />
         </Modal>
       </StrictMode>,