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/hooks/useDebouncedValidation.ts

@@ -9,16 +9,19 @@ import { useEffect, useRef, useState } from "react";
  * @param delay - Debounce delay in milliseconds (default: 500)
  * @returns Object containing current errors and a function to manually trigger validation
  */
-export function useDebouncedValidation<T>(
+export function useDebouncedValidation<
+  T,
+  E extends Record<string, string | undefined> = Record<string, string>,
+>(
   formData: T,
-  validator: (data: T) => Record<string, string>,
+  validator: (data: T) => E,
   delay: number = 500
 ): {
-  errors: Record<string, string>;
-  setErrors: React.Dispatch<React.SetStateAction<Record<string, string>>>;
+  errors: E;
+  setErrors: React.Dispatch<React.SetStateAction<E>>;
   validate: (data?: T) => void;
 } {
-  const [errors, setErrors] = useState<Record<string, string>>({});
+  const [errors, setErrors] = useState<E>({} as E);
   const validationTimeoutRef = useRef<NodeJS.Timeout | null>(null);
   const formDataRef = useRef<T>(formData);