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/desktop/tests/unit/payment-handlers.spec.ts

@@ -85,8 +85,8 @@ describe('desktop payment IPC security boundary', () => {
     expect(name).toBe('stripe-checkout')
     expect(body).toEqual({
       tier: 'pro',
-      success_url: 'https://d3ro.chanpaca.net/billing?desktop_checkout=success',
-      cancel_url: 'https://d3ro.chanpaca.net/billing?desktop_checkout=cancelled',
+      success_url: 'https://d3ro.chanpaca.net/app/billing?success=1',
+      cancel_url: 'https://d3ro.chanpaca.net/app/billing?canceled=1',
       idempotency_key: expect.stringMatching(/^desktop:stripe-checkout:[0-9a-f-]{36}$/)
     })
     expect(options).toMatchObject({ timeoutMs: PAYMENT_REQUEST_TIMEOUT_MS })