Let a lone app widget fill its record-page tab edge to edge (extend #27863 to record pages)

Context Since v2.49, app widgets on record pages no longer have their own frame (#28146: "Apps that need a border around their content draw it themselves"). #27863 also lets a lone widget on a standalone page drop the card frame and its padding, so the app fills the page edge to edge. Record pages were explicitly left out of #27863. A record-page tab that contains a single app (front component) widget still renders it inside the widget card, with the card's padding, even though there is nothing else on the tab to separate it from. Why this matters Many apps use a record-page tab as a full working area: a kanban board, a pipeline, a timeline, a data table. The tab is configured as VERTICAL_LIST with one FRONT_COMPONENT widget set to heightBehavior: TAB_VIEWPORT, so the widget already takes the whole tab height. • Before page layouts (≤ v2.36), such tabs rendered edge to edge, which suited these UIs well. • Since v2.37 they sit inside a padded card. Until v…