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.

app/(main)/admin/_tabs/branding.tsx

@@ -33,6 +33,8 @@ const TEXT_FIELDS = [
 
 const PWA_IMAGE_FIELDS = [
   { key: 'pwaIconUrl', label: 'PWA Icon', accept: '.svg,.png,.jpg,.webp' },
+  { key: 'pwaScreenshotMobileUrl', label: 'PWA Screenshot (Mobile)', accept: '.png,.jpg,.webp' },
+  { key: 'pwaScreenshotDesktopUrl', label: 'PWA Screenshot (Desktop)', accept: '.png,.jpg,.webp' },
 ] as const;
 
 const PWA_TEXT_FIELDS = [