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;

lib/admin/types.ts

@@ -142,6 +142,8 @@ export const CONFIG_ENV_MAP: Record<string, { envVar: string; fileEnvVar?: strin
   devMode: { envVar: 'DEV_MOCK_JMAP', type: 'boolean', defaultValue: false },
   faviconUrl: { envVar: 'FAVICON_URL', type: 'url', defaultValue: '/branding/Bulwark_Favicon.svg' },
   pwaIconUrl: { envVar: 'PWA_ICON_URL', type: 'url', defaultValue: '' },
+  pwaScreenshotMobileUrl: { envVar: 'PWA_SCREENSHOT_MOBILE_URL', type: 'url', defaultValue: '' },
+  pwaScreenshotDesktopUrl: { envVar: 'PWA_SCREENSHOT_DESKTOP_URL', type: 'url', defaultValue: '' },
   pwaThemeColor: { envVar: 'PWA_THEME_COLOR', type: 'string', defaultValue: '#ffffff' },
   pwaBackgroundColor: { envVar: 'PWA_BACKGROUND_COLOR', type: 'string', defaultValue: '#ffffff' },
   appLogoLightUrl: { envVar: 'APP_LOGO_LIGHT_URL', type: 'url', defaultValue: '' },