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/web/src/components/billing/portal-button.tsx

@@ -7,6 +7,7 @@ import { useState } from 'react'
 import { Button, CircularProgress, Alert, Box } from '@mui/material'
 import SettingsIcon from '@mui/icons-material/Settings'
 import { getSupabaseBrowserClient } from '@/lib/supabase-browser'
+import { browserBillingUrl } from '@/lib/web-app-url'
 
 export function PortalButton(): React.ReactElement {
   const [busy, setBusy] = useState(false)
@@ -39,7 +40,7 @@ export function PortalButton(): React.ReactElement {
             'Content-Type': 'application/json'
           },
           body: JSON.stringify({
-            return_url: `${window.location.origin}/billing`
+            return_url: browserBillingUrl()
           })
         }
       )