fix(web): keep onboarding and mobile natal board on one aligned overlay

Put the compact board trigger in the chat header, keep the sheet above the composer on iOS, and align onboarding messages with the intake card on the session column.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/components/rectification-board.tsx

@@ -271,6 +271,7 @@ export function RectificationBoardPeek({
     <button
       className="rectification-board-peek"
       type="button"
+      aria-label={peek.detail ? `${peek.title}，${peek.detail}` : peek.title}
       aria-expanded={expanded}
       aria-controls={boardId}
       onClick={onOpen}