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/admin/price-history/page.tsx

@@ -2,7 +2,7 @@
 
 import { useEffect, useState, useCallback } from "react";
 import { Permission } from "../../auth-context";
-import { api } from "../../api";
+import { adminApi } from "../../api";
 import { sharedStyles } from "../../styles/shared";
 import { Header } from "../../components/Header";
 import { useRequireAuth } from "../../hooks/useRequireAuth";
@@ -24,7 +24,7 @@ export default function AdminPriceHistoryPage() {
     setError(null);
     setIsLoadingData(true);
     try {
-      const data = await api.get<PriceHistoryRecord[]>("/api/audit/price-history");
+      const data = await adminApi.getPriceHistory();
       setRecords(data);
     } catch (err) {
       setRecords([]);
@@ -38,7 +38,7 @@ export default function AdminPriceHistoryPage() {
     setIsFetching(true);
     setError(null);
     try {
-      await api.post<PriceHistoryRecord>("/api/audit/price-history/fetch", {});
+      await adminApi.fetchPrice();
       await fetchRecords();
     } catch (err) {
       setError(err instanceof Error ? err.message : "Failed to fetch price");