A forge preview's look pictures every top-level page, desktop and phone, light and dark #2708

For #2678 and the owner's rule that every page change carries before/after screenshots. Builders cannot take screenshots in their sandbox; the forge's look service (src/looks/service.ts, src/looks/capture.ts) can, on the preview.

Done:

• [ ] one module lists the forge's top-level pages (/, /map, /boards, /system, /flow, /usage and the rest the router serves at the top level), and the look of the forge's own preview pictures each at desktop and 360px, in light and dark
• [ ] the pull page shows them grouped by page, beside the same page's look from main (before/after)
• [ ] a look that could not be taken is a row that says why, as today
• [ ] test: the page list matches the router's top-level routes, and a preview look records one picture per page × width × theme

Files: src/looks/*, src/web/pages/looks.ts. Risk: capture time per preview; keep it best-effort, never blocking a check.

State: split. A builder left this without a pull because it asked for a page list, two themes and a before/after view in one run. forge.json's app.looks already picks which pages a look pictures, so the fresh issues are: list every top-level page there (#2735), picture dark as well as light (#2736), and show each pull look beside main's look of the same page (#2737).