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/profile/page.tsx

@@ -2,7 +2,7 @@
 
 import { useEffect, useState, useCallback } from "react";
 
-import { api } from "../api";
+import { profileApi } from "../api";
 import { extractApiErrorMessage, extractFieldErrors } from "../utils/error-handling";
 import { Permission } from "../auth-context";
 import { Header } from "../components/Header";
@@ -81,7 +81,7 @@ export default function ProfilePage() {
 
   const fetchProfile = useCallback(async () => {
     try {
-      const data = await api.get<ProfileData>("/api/profile");
+      const data = await profileApi.getProfile();
       const formValues = toFormData(data);
       setFormData(formValues);
       setOriginalData(formValues);
@@ -132,7 +132,7 @@ export default function ProfilePage() {
     setIsSubmitting(true);
 
     try {
-      const data = await api.put<ProfileData>("/api/profile", {
+      const data = await profileApi.updateProfile({
         contact_email: formData.contact_email || null,
         telegram: formData.telegram || null,
         signal: formData.signal || null,