feat(web): serve the web app under /app and send every billing link there (WS-B)

apps/web was never deployed, so /billing on the public domain returned the
landing page and d3ro.dev (desktop "upgrade") did not resolve.

- apps/web runs with basePath /app and output standalone; /download and
  /releases redirect to the site's #download. A Dockerfile and a d3ro-web
  compose service (port 3002) deploy it to the NAS with the other images.
- The site bridge worker forwards /app/* to WEB_APP_ORIGIN (the tunnel host)
  and rewrites upstream redirects; everything else still goes to Pages.
  With no origin configured /app answers 503 instead of the landing page.
- Desktop upgrade, desktop Stripe return, mobile subscription management,
  the web checkout/portal returns and the site all use billingUrl(); the
  return query is success=1 / canceled=1, which the billing page reads.
  The billing page highlights ?tier=pro|pro_plus, and signing in from a
  billing link returns to the same plan.
- auth/callback pins the redirect origin in production and rejects
  protocol-relative next= values (open redirect).
- Mobile legal links use SITE_URLS (fixes the missing slash on /terms).
- Compose drops the unused NEXT_PUBLIC_API_URL and the dead wwwroot legal
  mounts; deploy scripts add the web image and the SUPABASE_* values the NAS
  compose already required; .dockerignore keeps app .env files out of images.
- Supabase auth redirects allow /app/** (remote dashboard must match).

Policy: docs/REFACTOR_POLICY.md Wave 3, W3-3 and W3-4.

apps/mobile-rn/src/screens/SettingsScreen.tsx

@@ -11,6 +11,7 @@ import {
 import { useSafeAreaInsets } from 'react-native-safe-area-context'
 import { useNavigation } from '@react-navigation/native'
 import { useI18n, type TranslationKey } from '@d3ro/i18n'
+import { SITE_URLS } from '@d3ro/core/web-urls'
 import { useAuth } from '../lib/auth-context'
 import { useEntitlement } from '../lib/entitlement-context'
 import {
@@ -475,21 +476,21 @@ export default function SettingsScreen(): React.ReactElement {
           label={t('mobile.set.privacyPolicy')}
           hint={t('mobile.set.privacyPolicyHint')}
           testID="settings-privacy-policy"
-          onPress={() => void Linking.openURL('https://d3ro.chanpaca.net/privacy/')}
+          onPress={() => void Linking.openURL(SITE_URLS.privacy)}
         />
         <LegalLinkRow
           label={t('mobile.set.termsOfService')}
           hint={t('mobile.set.termsOfServiceHint')}
           testID="settings-terms-of-service"
           bordered
-          onPress={() => void Linking.openURL('https://d3ro.chanpaca.net/terms/')}
+          onPress={() => void Linking.openURL(SITE_URLS.terms)}
         />
         <LegalLinkRow
           label={t('mobile.set.accountDeletion')}
           hint={t('mobile.set.accountDeletionHint')}
           testID="settings-account-deletion-web"
           bordered
-          onPress={() => void Linking.openURL('https://d3ro.chanpaca.net/delete-account/')}
+          onPress={() => void Linking.openURL(SITE_URLS.deleteAccount)}
         />
       </ThemeCard>