Refactor API layer into structured domain-specific modules

- Created new api/ directory with domain-specific API modules:
  - api/client.ts: Base API client with error handling
  - api/auth.ts: Authentication endpoints
  - api/exchange.ts: Exchange/price endpoints
  - api/trades.ts: User trade endpoints
  - api/profile.ts: Profile management endpoints
  - api/invites.ts: Invite endpoints
  - api/admin.ts: Admin endpoints
  - api/index.ts: Centralized exports

- Migrated all API calls from ad-hoc api.get/post/put to typed domain APIs
- Updated all imports across codebase
- Fixed test mocks to use new API structure
- Fixed type issues in validation utilities
- Removed old api.ts file

Benefits:
- Type-safe endpoints (no more string typos)
- Centralized API surface (easy to discover endpoints)
- Better organization (domain-specific modules)
- Uses generated OpenAPI types automatically

frontend/app/api.ts

@@ -1,67 +0,0 @@
-import { API_URL } from "./config";
-
-/**
- * Simple API client that centralizes fetch configuration.
- * All requests include credentials and proper headers.
- */
-
-export class ApiError extends Error {
-  constructor(
-    message: string,
-    public status: number,
-    public data?: unknown
-  ) {
-    super(message);
-    this.name = "ApiError";
-  }
-}
-
-async function request<T>(endpoint: string, options: RequestInit = {}): Promise<T> {
-  const url = `${API_URL}${endpoint}`;
-
-  const headers: Record<string, string> = {
-    ...(options.headers as Record<string, string>),
-  };
-
-  if (options.body && typeof options.body === "string") {
-    headers["Content-Type"] = "application/json";
-  }
-
-  const res = await fetch(url, {
-    ...options,
-    headers,
-    credentials: "include",
-  });
-
-  if (!res.ok) {
-    let data: unknown;
-    try {
-      data = await res.json();
-    } catch {
-      // Response wasn't JSON
-    }
-    throw new ApiError(`Request failed: ${res.status}`, res.status, data);
-  }
-
-  return res.json();
-}
-
-export const api = {
-  get<T>(endpoint: string): Promise<T> {
-    return request<T>(endpoint);
-  },
-
-  post<T>(endpoint: string, body?: unknown): Promise<T> {
-    return request<T>(endpoint, {
-      method: "POST",
-      body: body ? JSON.stringify(body) : undefined,
-    });
-  },
-
-  put<T>(endpoint: string, body?: unknown): Promise<T> {
-    return request<T>(endpoint, {
-      method: "PUT",
-      body: body ? JSON.stringify(body) : undefined,
-    });
-  },
-};