feat(ui): generalize billing provider descriptors and payment method display

[yyyyaaa]

Makes the billing templates provider-neutral in shape, not just in name, and stops showing provider internals to customers.

• Descriptor fields on BillingProviderDescriptor (all optional, so existing descriptors keep working):
  • logo: an image drawn on the provider tile instead of the monogram
  • merchantOfRecord: the payment panel says the provider charges, handles tax, and issues invoices
  • immutablePrices: the catalog copy about immutable prices only appears for providers that work that way
  • modeLabels: e.g. Sandbox instead of Test, used by the console badge, sidebar, provider card, and feature chip
  • detectMode: replaces running Stripe's key-prefix parsing on every provider's credentials
• Customer invoices view: drops the raw sub_… id and the merchant-dashboard link, which customers couldn't open, and shows the method on file through a new provider-neutral paymentMethod field rendered by PaymentMethodRow (card brand, last four, expiry with "Expires soon" / expired states; wallets, bank accounts, invoicing).
• Demo data: payment methods on the account fixtures (an expired card in the overdue scenario), and the example Paddle and Lemon Squeezy descriptors marked as merchant of record.

const PADDLE: BillingProviderDescriptor = {
  ...base,
  merchantOfRecord: true,
  modeLabels: { test: 'Sandbox', live: 'Live' },
  detectMode: (values) => (values.PADDLE_API_KEY?.includes('_sdbx_') ? 'test' : 'live'),
};

Before	After
"Manage in Stripe" panel with the raw subscription id and a dashboard link	Method on file (brand, last four, expiry) and provider-specific copy; see Storybook Templates/Billing Account → Merchant of record provider and Billing Kit/History → PaymentMethodRow

🤖 Generated with Claude Code

https://claude.ai/code/session_01GPTLDuvMBGBawVLKL42yYV

[tenki-reviewer]

Review complete. No blocking issues — approved ✅; 1 nitpick below.

🧹 Nitpicks (1) — 🟢 1 low

• 🟢 Broken logo URL leaves no monogram fallback (provider.tsx:41) — ProviderMark renders <img src={provider.logo}> whenever logo is set, with no load-error fallback to the monogram (packages/ui/src/components/billing-kit/provider.tsx:41).

---

The change reworks how billing providers describe themselves and how payment methods render to customers. Provider descriptors gain explicit mode-detection and label fallbacks, and the console's connect flow now calls detectMode on submitted values instead of guessing from a secret-prefix heuristic. Customer-facing invoice views drop provider object ids and dashboard links in favor of display-grade payment fields (icon, brand, last4, expiry state via a new UTC-safe monthsToExpiry helper).

Files	Change
packages/ui/src/components/billing-kit/{providers,provider,types,demo,index}	Provider descriptor contract: detectMode, mode/feature labels, merchant-of-record and immutable-price flags, logo handling
packages/ui/src/components/billing-kit/invoices.tsx	Payment-method summary rendering with expiry month math and tone mapping
packages/ui/src/components/billing-console/*	Connect flow and catalog/provider views updated to the new descriptor contract
packages/ui/src/components/billing-account/*	Customer-facing invoice view: privacy-safe provider copy and fixtures
packages/ui/src/stories/billing/*, packages/ui/test/billing-templates.test.tsx	Story variants and tests covering expiry boundaries and privacy assertions

_{Reviewed commit: c0d2387}