Feature: configurable PWA install screenshots (per-domain)

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.

lib/admin/domain-branding.ts

@@ -15,6 +15,8 @@ export const BRANDING_OVERRIDE_KEYS = [
   'appDescription',
   'faviconUrl',
   'pwaIconUrl',
+  'pwaScreenshotMobileUrl',
+  'pwaScreenshotDesktopUrl',
   'pwaThemeColor',
   'pwaBackgroundColor',
   'appLogoLightUrl',
@@ -42,6 +44,8 @@ export interface DomainBrandingEntry {
   appDescription?: string;
   faviconUrl?: string;
   pwaIconUrl?: string;
+  pwaScreenshotMobileUrl?: string;
+  pwaScreenshotDesktopUrl?: string;
   pwaThemeColor?: string;
   pwaBackgroundColor?: string;
   appLogoLightUrl?: string;