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/exchange/hooks/useAvailableSlots.ts

@@ -1,10 +1,9 @@
 import { useState, useEffect, useCallback } from "react";
-import { api } from "../../api";
+import { exchangeApi } from "../../api";
 import { components } from "../../generated/api";
 import { formatDate } from "../../utils/date";
 
 type BookableSlot = components["schemas"]["BookableSlot"];
-type AvailableSlotsResponse = components["schemas"]["AvailableSlotsResponse"];
 
 interface UseAvailableSlotsOptions {
   /** Whether the user is authenticated and authorized */
@@ -48,7 +47,7 @@ export function useAvailableSlots(options: UseAvailableSlotsOptions): UseAvailab
 
       try {
         const dateStr = formatDate(date);
-        const data = await api.get<AvailableSlotsResponse>(`/api/exchange/slots?date=${dateStr}`);
+        const data = await exchangeApi.getSlots(dateStr);
         setAvailableSlots(data.slots);
       } catch (err) {
         console.error("Failed to fetch slots:", err);
@@ -70,7 +69,7 @@ export function useAvailableSlots(options: UseAvailableSlotsOptions): UseAvailab
       const promises = dates.map(async (date) => {
         try {
           const dateStr = formatDate(date);
-          const data = await api.get<AvailableSlotsResponse>(`/api/exchange/slots?date=${dateStr}`);
+          const data = await exchangeApi.getSlots(dateStr);
           if (data.slots.length > 0) {
             availabilitySet.add(dateStr);
           }