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/api/trades.ts

@@ -0,0 +1,30 @@
+import { client } from "./client";
+import { components } from "../generated/api";
+
+type ExchangeResponse = components["schemas"]["ExchangeResponse"];
+
+/**
+ * Trades API endpoints (user-facing)
+ */
+export const tradesApi = {
+  /**
+   * Get all trades for the current user
+   */
+  getTrades(): Promise<ExchangeResponse[]> {
+    return client.get<ExchangeResponse[]>("/api/trades");
+  },
+
+  /**
+   * Get a specific trade by public ID
+   */
+  getTrade(publicId: string): Promise<ExchangeResponse> {
+    return client.get<ExchangeResponse>(`/api/trades/${encodeURIComponent(publicId)}`);
+  },
+
+  /**
+   * Cancel a trade
+   */
+  cancelTrade(publicId: string): Promise<ExchangeResponse> {
+    return client.post<ExchangeResponse>(`/api/trades/${encodeURIComponent(publicId)}/cancel`, {});
+  },
+};