feat(billing): add /account/billing and /account/billing/select-plan pages

Wires up the missing frontend billing surfaces that TrialPill, UpgradePrompt,
NextStepCard, and SetupChecklist all link to. Trial-expired, canceled,
past-due, and "Pick a plan" CTAs no longer 404.

- BillingPage: subscription summary, status-specific messaging
  (trialing / past_due / canceled / complimentary), Manage billing button
  routed through the Stripe Customer Portal, and a Pick/Change-plan link.
- SelectPlanPage: plan picker with monthly/annual toggle + seat count.
  Starter/Pro hit /billing/checkout-session; Enterprise links to
  /contact-sales. Active current plan is tagged "Current plan" with a
  disabled CTA.
- billingApi.getPortalSession + createCheckoutSession; getPortalSession
  surfaces a typed BillingPortalError (no_stripe_customer / stripe_not_
  configured) so the UI can show the right toast.
- AccountSettingsPage gets a Billing link card so the page is discoverable
  from the account hub.
- 10 new vitest cases covering subscription summary, trial/past-due/
  canceled/complimentary states, portal-session error fallback, plan-card
  rendering, checkout payload, and current-plan badge.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/pages/account/__tests__/SelectPlanPage.test.tsx

@@ -0,0 +1,178 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { render, screen, fireEvent, waitFor } from '@testing-library/react'
+import { MemoryRouter } from 'react-router-dom'
+
+import { SelectPlanPage } from '../SelectPlanPage'
+import { useBillingStore } from '@/store/billingStore'
+
+vi.mock('@/api/billing', () => ({
+  billingApi: {
+    createCheckoutSession: vi.fn(),
+  },
+}))
+vi.mock('@/api/plans', () => ({
+  plansApi: {
+    getPublic: vi.fn(),
+  },
+}))
+vi.mock('@/lib/toast', () => ({
+  toast: {
+    error: vi.fn(),
+    success: vi.fn(),
+  },
+}))
+
+import { billingApi } from '@/api/billing'
+import { plansApi } from '@/api/plans'
+
+const createCheckoutSession = billingApi.createCheckoutSession as unknown as ReturnType<typeof vi.fn>
+const getPublic = plansApi.getPublic as unknown as ReturnType<typeof vi.fn>
+
+const PLAN_FIXTURE = [
+  {
+    plan: 'starter',
+    display_name: 'Starter',
+    description: 'For solo techs.',
+    monthly_price_cents: 1900,
+    annual_price_cents: 19000,
+    max_seats: 1,
+    sort_order: 1,
+    is_public: true,
+  },
+  {
+    plan: 'pro',
+    display_name: 'Pro',
+    description: 'For growing teams.',
+    monthly_price_cents: 4900,
+    annual_price_cents: 49000,
+    max_seats: 5,
+    sort_order: 2,
+    is_public: true,
+  },
+  {
+    plan: 'enterprise',
+    display_name: 'Enterprise',
+    description: 'Custom.',
+    monthly_price_cents: null,
+    annual_price_cents: null,
+    max_seats: null,
+    sort_order: 3,
+    is_public: true,
+  },
+]
+
+function renderPage() {
+  return render(
+    <MemoryRouter>
+      <SelectPlanPage />
+    </MemoryRouter>,
+  )
+}
+
+describe('SelectPlanPage', () => {
+  // Stub window.location.href setter so we can assert without a real navigation.
+  let assignedHref: string | null = null
+  const originalLocation = window.location
+
+  beforeEach(() => {
+    getPublic.mockReset()
+    createCheckoutSession.mockReset()
+    assignedHref = null
+    Object.defineProperty(window, 'location', {
+      configurable: true,
+      value: {
+        ...originalLocation,
+        get href() {
+          return assignedHref ?? originalLocation.href
+        },
+        set href(v: string) {
+          assignedHref = v
+        },
+      },
+    })
+    useBillingStore.setState({
+      subscription: null,
+      planBilling: null,
+      planLimits: {},
+      enabledFeatures: {},
+      isLoading: false,
+      error: null,
+    })
+  })
+
+  it('renders plan cards from plansApi', async () => {
+    getPublic.mockResolvedValueOnce(PLAN_FIXTURE)
+    renderPage()
+
+    await waitFor(() => {
+      expect(screen.getByTestId('plan-card-starter')).toBeInTheDocument()
+    })
+    expect(screen.getByTestId('plan-card-pro')).toBeInTheDocument()
+    expect(screen.getByTestId('plan-card-enterprise')).toBeInTheDocument()
+  })
+
+  it('Continue to checkout calls createCheckoutSession and redirects', async () => {
+    getPublic.mockResolvedValueOnce(PLAN_FIXTURE)
+    createCheckoutSession.mockResolvedValueOnce({ url: 'https://checkout.stripe.com/abc' })
+
+    renderPage()
+    await waitFor(() => {
+      expect(screen.getByTestId('plan-card-pro')).toBeInTheDocument()
+    })
+
+    // Bump seats and switch to annual.
+    fireEvent.change(screen.getByTestId('seats-input'), { target: { value: '3' } })
+    fireEvent.click(screen.getByTestId('interval-annual'))
+
+    fireEvent.click(screen.getByTestId('plan-cta-pro'))
+
+    await waitFor(() => {
+      expect(createCheckoutSession).toHaveBeenCalledWith({
+        plan: 'pro',
+        seats: 3,
+        billing_interval: 'annual',
+      })
+    })
+    await waitFor(() => {
+      expect(assignedHref).toBe('https://checkout.stripe.com/abc')
+    })
+  })
+
+  it('Talk to sales links to /contact-sales for enterprise', async () => {
+    getPublic.mockResolvedValueOnce(PLAN_FIXTURE)
+    renderPage()
+    await waitFor(() => {
+      expect(screen.getByTestId('plan-cta-enterprise')).toBeInTheDocument()
+    })
+    const cta = screen.getByTestId('plan-cta-enterprise') as HTMLAnchorElement
+    expect(cta.getAttribute('href')).toBe('/contact-sales')
+  })
+
+  it('marks the active current plan as Current plan and disables its CTA', async () => {
+    getPublic.mockResolvedValueOnce(PLAN_FIXTURE)
+    useBillingStore.setState({
+      subscription: {
+        status: 'active',
+        plan: 'pro',
+        current_period_start: '2026-04-01T00:00:00Z',
+        current_period_end: '2026-05-01T00:00:00Z',
+        cancel_at_period_end: false,
+        seat_limit: 5,
+        has_pro_entitlement: true,
+        is_paid: true,
+      },
+      planBilling: null,
+      planLimits: {},
+      enabledFeatures: {},
+      isLoading: false,
+      error: null,
+    })
+
+    renderPage()
+    await waitFor(() => {
+      expect(screen.getByTestId('plan-current-pro')).toBeInTheDocument()
+    })
+    const cta = screen.getByTestId('plan-cta-pro') as HTMLButtonElement
+    expect(cta).toBeDisabled()
+  })
+})