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/api/setup/branding/route.ts

@@ -23,10 +23,13 @@ const ALLOWED_MIME_TYPES = new Set([
 
 const VALID_SLOTS = new Set([
   'faviconUrl',
+  'pwaIconUrl',
   'appLogoLightUrl',
   'appLogoDarkUrl',
   'loginLogoLightUrl',
   'loginLogoDarkUrl',
+  'pwaScreenshotMobileUrl',
+  'pwaScreenshotDesktopUrl',
 ]);
 
 const EXT_BY_MIME: Record<string, string> = {