Admins can upload custom mobile/desktop screenshots shown in the browser's
PWA install dialog, replacing the hardcoded Bulwark ones. Two new config keys
(pwaScreenshotMobileUrl/DesktopUrl), upload widgets in the admin Branding tab,
a sharp-based /api/pwa-screenshot/[variant] resize route, and manifest.ts picks
the custom screenshots when configured.
Like the other branding fields, screenshots are per-domain: they are
BRANDING_OVERRIDE_KEYS, the manifest and the /api/pwa-screenshot route resolve
them from the request host (domain override -> global -> Bulwark default), and
the admin Branding tab + upload/delete route handle them in a per-domain scope,
mirroring pwaIconUrl/faviconUrl.
