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/ProPaywallScreen.tsx

@@ -24,6 +24,7 @@ import {
 } from '../lib/mobile-ads-context'
 import { useMobilePreferences } from '../lib/preferences-context'
 import { ThemeButton, ThemeCard, ThemeText } from '../theme/themed-components'
+import { SITE_URLS } from '@d3ro/core/web-urls'
 
 type Navigation = NativeStackNavigationProp<RootStackParamList, 'ProPaywall'>
 type TranslationKey = Parameters<ReturnType<typeof useI18n>['t']>[0]
@@ -305,7 +306,7 @@ export default function ProPaywallScreen(): React.ReactElement {
           <Pressable
             accessibilityRole="link"
             accessibilityLabel={t('mobile.paywall.terms')}
-            onPress={() => void Linking.openURL('https://d3ro.chanpaca.net/terms')}
+            onPress={() => void Linking.openURL(SITE_URLS.terms)}
             style={styles.legalLink}
           >
             <ThemeText variant="small" color="accent">{t('mobile.paywall.terms')}</ThemeText>
@@ -314,7 +315,7 @@ export default function ProPaywallScreen(): React.ReactElement {
           <Pressable
             accessibilityRole="link"
             accessibilityLabel={t('mobile.paywall.privacy')}
-            onPress={() => void Linking.openURL('https://d3ro.chanpaca.net/privacy/')}
+            onPress={() => void Linking.openURL(SITE_URLS.privacy)}
             style={styles.legalLink}
           >
             <ThemeText variant="small" color="accent">{t('mobile.paywall.privacy')}</ThemeText>