BillingHub: handle comped accounts gracefully

[AutumnsGrove]

Summary

Comped accounts (oak/evergreen with no Stripe customer) get a BILLING-040 error when redirected to the BillingHub portal or cancel pages. The portal page (billing.grove.place/portal) calls proxyToBillingApi("/portal") which fails because there's no Stripe customer record to create a portal session for.

Steps to Reproduce

1. Be a comped account (e.g., admin/dev with Evergreen plan, no Stripe subscription)
2. Go to /arbor/account
3. Click the settings cog on GardenStatus → redirects to billing.grove.place/portal
4. See: 502 error with code BILLING-040 "Could not open the billing portal"

Same for clicking Cancel Membership → billing.grove.place/cancel.

Expected Behavior

BillingHub should detect comped accounts and show a friendly message like "Your account is complimentary — no billing to manage" instead of crashing.

Current Workaround

Cancel/Resume buttons are hidden for comped accounts on the Account page (bb4481f07). But the settings cog and nurture CTA still redirect to BillingHub which errors.

Files to Investigate

• apps/billing/src/routes/portal/+page.server.ts — portal session creation (line 42-57)
• apps/billing/src/routes/cancel/+page.svelte — cancel flow
• apps/billing/src/lib/billing-proxy.ts — API proxy

Acceptance Criteria

• Portal page shows a helpful message for comped accounts instead of 502
• Cancel page handles comped accounts (no subscription to cancel)
• Resume page handles comped accounts similarly