PR #3415 - ReviewStage/luke

What

A finite frame variant for CollectionPage (F37) and DetailPage (F38), so desktop screens that sit under the window's toolbar (Issues, Inbox, Factory, Automations, an issue's page) can adopt them. Found by D37 (#3414 stack). No screen migrates here; D41+ adopt it.

PAGE_FRAME = { PAGE, TOOLBAR } (new page-frame module, exported from @luke/ui). PAGE is the default, so every existing caller is unchanged.
- TOOLBAR: no in-page title header. The caller hands its window toolbar in toolbar (optional, for a window that draws it elsewhere), which the page stands still above its scroll, and a label: string, the toolbar's title, which names the region (aria-label). title/description/actions are typed never under it, so the two headings cannot be mixed.
- The page still owns content width, the loading/empty/error states, filters (CollectionPage), the secondary column (DetailPage), and the scroll boundary below the toolbar. Content opens a section gap (16px) below the toolbar's hairline, inside the scroll.
PAGE_WIDTH = { MEASURE, FULL }. MEASURE is the default. FULL stands content across the room at the page inset and lets the content scroll sideways, for a board.

Visible changes

None for existing callers. The new variants are shown only in the catalog's new sections ("Under the window's toolbar" on both pages, and "A board across the room, scrolling sideways" on the collection page). The catalog's toolbar is a stand-in for the desktop's.

Tests

jsdom, through the public API: under the toolbar the region is named by its label, has no h1 of its own, and shows the toolbar and its act. Every collection state shows what it says, and the toolbar and filters stay the same elements across state changes. The detail page keeps its secondary landmark.