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/invites/page.tsx

@@ -1,7 +1,7 @@
 "use client";
 
 import { useEffect, useState, useCallback } from "react";
-import { api } from "../api";
+import { invitesApi } from "../api";
 import { Header } from "../components/Header";
 import { useRequireAuth } from "../hooks/useRequireAuth";
 import { components } from "../generated/api";
@@ -29,7 +29,7 @@ export default function InvitesPage() {
 
   const fetchInvites = useCallback(async () => {
     try {
-      const data = await api.get<Invite[]>("/api/invites");
+      const data = await invitesApi.getInvites();
       setInvites(data);
     } catch (err) {
       console.error("Failed to load invites:", err);