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/auth-context.tsx

@@ -2,7 +2,7 @@
 
 import { createContext, useContext, useState, useEffect, useCallback, ReactNode } from "react";
 
-import { api } from "./api";
+import { authApi } from "./api";
 import { components } from "./generated/api";
 import { extractApiErrorMessage } from "./utils/error-handling";
 
@@ -54,7 +54,7 @@ export function AuthProvider({ children }: { children: ReactNode }) {
 
   const checkAuth = async () => {
     try {
-      const userData = await api.get<User>("/api/auth/me");
+      const userData = await authApi.getMe();
       setUser(userData);
     } catch {
       // Not authenticated
@@ -65,7 +65,7 @@ export function AuthProvider({ children }: { children: ReactNode }) {
 
   const login = async (email: string, password: string) => {
     try {
-      const userData = await api.post<User>("/api/auth/login", { email, password });
+      const userData = await authApi.login(email, password);
       setUser(userData);
     } catch (err) {
       throw new Error(extractApiErrorMessage(err, "Login failed"));
@@ -74,11 +74,7 @@ export function AuthProvider({ children }: { children: ReactNode }) {
 
   const register = async (email: string, password: string, inviteIdentifier: string) => {
     try {
-      const userData = await api.post<User>("/api/auth/register", {
-        email,
-        password,
-        invite_identifier: inviteIdentifier,
-      });
+      const userData = await authApi.register(email, password, inviteIdentifier);
       setUser(userData);
     } catch (err) {
       throw new Error(extractApiErrorMessage(err, "Registration failed"));
@@ -87,7 +83,7 @@ export function AuthProvider({ children }: { children: ReactNode }) {
 
   const logout = async () => {
     try {
-      await api.post("/api/auth/logout");
+      await authApi.logout();
     } catch {
       // Ignore errors on logout
     }