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/availability/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 { Header } from "../../components/Header";
 import { useRequireAuth } from "../../hooks/useRequireAuth";
 import { components } from "../../generated/api";
@@ -25,7 +25,6 @@ import {
 const { slotDurationMinutes, maxAdvanceDays, minAdvanceDays } = constants.exchange;
 
 type _AvailabilityDay = components["schemas"]["AvailabilityDay"];
-type AvailabilityResponse = components["schemas"]["AvailabilityResponse"];
 type TimeSlot = components["schemas"]["TimeSlot"];
 
 // Generate time options for dropdowns (15-min intervals)
@@ -73,9 +72,7 @@ export default function AdminAvailabilityPage() {
     try {
       const fromDate = formatDate(dateRange[0]);
       const toDate = formatDate(dateRange[dateRange.length - 1]);
-      const data = await api.get<AvailabilityResponse>(
-        `/api/admin/availability?from=${fromDate}&to=${toDate}`
-      );
+      const data = await adminApi.getAvailability(fromDate, toDate);
 
       const map = new Map<string, TimeSlot[]>();
       for (const day of data.days) {
@@ -140,7 +137,7 @@ export default function AdminAvailabilityPage() {
         end_time: s.end_time + ":00",
       }));
 
-      await api.put("/api/admin/availability", {
+      await adminApi.updateAvailability({
         date: formatDate(selectedDate),
         slots,
       });
@@ -161,7 +158,7 @@ export default function AdminAvailabilityPage() {
     setError(null);
 
     try {
-      await api.put("/api/admin/availability", {
+      await adminApi.updateAvailability({
         date: formatDate(selectedDate),
         slots: [],
       });
@@ -203,7 +200,7 @@ export default function AdminAvailabilityPage() {
     setError(null);
 
     try {
-      await api.post("/api/admin/availability/copy", {
+      await adminApi.copyAvailability({
         source_date: copySource,
         target_dates: Array.from(copyTargets),
       });