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/api/billing.ts

@@ -1,5 +1,15 @@
+import { AxiosError } from 'axios'
+
 import apiClient from './client'
-import type { BillingStateApiResponse, BillingStatePayload } from '@/types'
+import {
+  BillingPortalError,
+  type BillingPortalErrorCode,
+  type BillingPortalSessionResponse,
+  type BillingStateApiResponse,
+  type BillingStatePayload,
+  type CheckoutSessionRequest,
+  type CheckoutSessionResponse,
+} from '@/types/billing'
 
 /**
  * Single boundary where the snake_case backend payload is transformed
@@ -22,6 +32,48 @@ export const billingApi = {
     const response = await apiClient.get<BillingStateApiResponse>('/billing/state')
     return transformBillingState(response.data)
   },
+
+  /**
+   * Request a Stripe Customer Portal session URL for the active account.
+   *
+   * Throws a typed `BillingPortalError` when:
+   * - HTTP 503 → `stripe_not_configured` (server-side Stripe is disabled)
+   * - HTTP 400 + `error: 'no_stripe_customer'` → account hasn't been billed yet
+   *
+   * Other errors (5xx, network) propagate as the underlying AxiosError.
+   */
+  async getPortalSession(): Promise<BillingPortalSessionResponse> {
+    try {
+      const response = await apiClient.get<BillingPortalSessionResponse>(
+        '/billing/portal-session',
+      )
+      return response.data
+    } catch (err) {
+      if (err instanceof AxiosError && err.response) {
+        const { status, data } = err.response
+        const code: BillingPortalErrorCode | null =
+          status === 503
+            ? 'stripe_not_configured'
+            : status === 400 && data?.detail?.error === 'no_stripe_customer'
+              ? 'no_stripe_customer'
+              : null
+        if (code) {
+          throw new BillingPortalError(code)
+        }
+      }
+      throw err
+    }
+  },
+
+  async createCheckoutSession(
+    payload: CheckoutSessionRequest,
+  ): Promise<CheckoutSessionResponse> {
+    const response = await apiClient.post<CheckoutSessionResponse>(
+      '/billing/checkout-session',
+      payload,
+    )
+    return response.data
+  },
 }
 
 export default billingApi