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/trades/page.tsx

@@ -2,7 +2,7 @@
 
 import { useEffect, useState, useCallback, CSSProperties } from "react";
 import { Permission } from "../../auth-context";
-import { api } from "../../api";
+import { adminApi } from "../../api";
 import { Header } from "../../components/Header";
 import { SatsDisplay } from "../../components/SatsDisplay";
 import { useRequireAuth } from "../../hooks/useRequireAuth";
@@ -47,7 +47,7 @@ export default function AdminTradesPage() {
 
   const fetchUpcomingTrades = useCallback(async (): Promise<string | null> => {
     try {
-      const data = await api.get<AdminExchangeResponse[]>("/api/admin/trades/upcoming");
+      const data = await adminApi.getUpcomingTrades();
       setUpcomingTrades(data);
       return null;
     } catch (err) {
@@ -58,21 +58,17 @@ export default function AdminTradesPage() {
 
   const fetchPastTrades = useCallback(async (): Promise<string | null> => {
     try {
-      let url = "/api/admin/trades/past";
-      const params = new URLSearchParams();
-
+      const params: { status?: string; user_search?: string } = {};
       if (statusFilter !== "all") {
-        params.append("status", statusFilter);
+        params.status = statusFilter;
       }
       if (userSearch.trim()) {
-        params.append("user_search", userSearch.trim());
+        params.user_search = userSearch.trim();
       }
 
-      if (params.toString()) {
-        url += `?${params.toString()}`;
-      }
-
-      const data = await api.get<AdminExchangeResponse[]>(url);
+      const data = await adminApi.getPastTrades(
+        Object.keys(params).length > 0 ? params : undefined
+      );
       setPastTrades(data);
       return null;
     } catch (err) {
@@ -105,12 +101,13 @@ export default function AdminTradesPage() {
     setError(null);
 
     try {
-      const endpoint =
-        action === "no_show"
-          ? `/api/admin/trades/${publicId}/no-show`
-          : `/api/admin/trades/${publicId}/${action}`;
-
-      await api.post<AdminExchangeResponse>(endpoint, {});
+      if (action === "complete") {
+        await adminApi.completeTrade(publicId);
+      } else if (action === "no_show") {
+        await adminApi.noShowTrade(publicId);
+      } else if (action === "cancel") {
+        await adminApi.cancelTrade(publicId);
+      }
       // Refetch trades - errors from fetch are informational, not critical
       const [upcomingErr, pastErr] = await Promise.all([fetchUpcomingTrades(), fetchPastTrades()]);
       const fetchErrors = [upcomingErr, pastErr].filter(Boolean);