feat(web): scaffold Next.js 16 dashboard with design system and auth client (#82)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/web/src/lib/api.ts

@@ -0,0 +1,47 @@
+const GATEWAY_URL = process.env['NEXT_PUBLIC_GATEWAY_URL'] ?? 'http://localhost:4000';
+
+export interface ApiRequestInit extends Omit<RequestInit, 'body'> {
+  body?: unknown;
+}
+
+export interface ApiError {
+  statusCode: number;
+  message: string;
+}
+
+/**
+ * Fetch wrapper for the Mosaic gateway API.
+ * Sends credentials (cookies) and JSON body automatically.
+ */
+export async function api<T>(path: string, init?: ApiRequestInit): Promise<T> {
+  const { body, headers: customHeaders, ...rest } = init ?? {};
+
+  const headers: Record<string, string> = {
+    Accept: 'application/json',
+    ...(customHeaders as Record<string, string>),
+  };
+
+  if (body !== undefined) {
+    headers['Content-Type'] = 'application/json';
+  }
+
+  const res = await fetch(`${GATEWAY_URL}${path}`, {
+    credentials: 'include',
+    ...rest,
+    headers,
+    body: body !== undefined ? JSON.stringify(body) : undefined,
+  });
+
+  if (!res.ok) {
+    const errorBody = (await res.json().catch(() => ({
+      statusCode: res.status,
+      message: res.statusText,
+    }))) as ApiError;
+    throw Object.assign(new Error(errorBody.message), {
+      statusCode: errorBody.statusCode,
+    });
+  }
+
+  if (res.status === 204) return undefined as T;
+  return (await res.json()) as T;
+}