feat(web): add billing/subscription client stub for Pro tier

- billing-client.ts: getMySubscription, cancelSubscription, getUsageSummary
- Calls platform-service /subscriptions + /usage endpoints

web/src/lib/billing-client.ts

@@ -0,0 +1,83 @@
+/**
+ * Billing / Subscription client stub for ChronoMind.
+ *
+ * Calls platform-service subscription + usage endpoints via the auth-api base URL.
+ * Client-side only — uses the stored auth token for authorization.
+ */
+
+import { PRODUCT_ID, getBaseUrl } from './auth-api';
+
+function getToken(): string | null {
+  if (typeof window === 'undefined') return null;
+  return localStorage.getItem('chronomind_access_token');
+}
+
+async function billingFetch<T>(path: string, options?: RequestInit): Promise<T> {
+  const token = getToken();
+  const res = await fetch(`${getBaseUrl()}${path}`, {
+    ...options,
+    headers: {
+      'Content-Type': 'application/json',
+      'x-product-id': PRODUCT_ID,
+      ...(token ? { Authorization: `Bearer ${token}` } : {}),
+      ...(options?.headers as Record<string, string>),
+    },
+  });
+  if (!res.ok) throw new Error(`Billing API error: ${res.status}`);
+  return res.json();
+}
+
+// ── Types ──────────────────────────────────────────────────────────
+
+export interface Subscription {
+  id: string;
+  productId: string;
+  userId: string;
+  plan: 'free' | 'pro';
+  status: 'active' | 'cancelled' | 'past_due' | 'trialing';
+  currentPeriodStart: string;
+  currentPeriodEnd: string;
+  cancelAtPeriodEnd: boolean;
+  stripeCustomerId?: string;
+  stripeSubscriptionId?: string;
+  createdAt: string;
+  updatedAt: string;
+}
+
+// ── Subscription API ───────────────────────────────────────────────
+
+export async function getMySubscription(userId: string): Promise<Subscription | null> {
+  try {
+    return await billingFetch<Subscription>(`/subscriptions/${userId}`);
+  } catch {
+    return null;
+  }
+}
+
+export async function cancelSubscription(userId: string): Promise<Subscription | null> {
+  try {
+    return await billingFetch<Subscription>(`/subscriptions/${userId}`, {
+      method: 'PUT',
+      body: JSON.stringify({ cancelAtPeriodEnd: true }),
+    });
+  } catch {
+    return null;
+  }
+}
+
+// ── Usage API ──────────────────────────────────────────────────────
+
+export interface UsageSummary {
+  timersCreated: number;
+  routinesCompleted: number;
+  focusMinutes: number;
+}
+
+export async function getUsageSummary(userId: string, days = 30): Promise<UsageSummary | null> {
+  try {
+    const data = await billingFetch<{ summary: UsageSummary }>(`/usage/summary?userId=${userId}&days=${days}`);
+    return data.summary as UsageSummary;
+  } catch {
+    return null;
+  }
+}