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/exchange.ts

@@ -0,0 +1,41 @@
+import { client } from "./client";
+import { components } from "../generated/api";
+
+type ExchangePriceResponse = components["schemas"]["ExchangePriceResponse"];
+type AvailableSlotsResponse = components["schemas"]["AvailableSlotsResponse"];
+type ExchangeResponse = components["schemas"]["ExchangeResponse"];
+
+interface CreateExchangeRequest {
+  slot_start: string;
+  direction: "buy" | "sell";
+  bitcoin_transfer_method: "onchain" | "lightning";
+  eur_amount: number;
+}
+
+/**
+ * Exchange API endpoints
+ */
+export const exchangeApi = {
+  /**
+   * Get current exchange price
+   */
+  getPrice(): Promise<ExchangePriceResponse> {
+    return client.get<ExchangePriceResponse>("/api/exchange/price");
+  },
+
+  /**
+   * Get available slots for a specific date
+   */
+  getSlots(date: string): Promise<AvailableSlotsResponse> {
+    return client.get<AvailableSlotsResponse>(
+      `/api/exchange/slots?date=${encodeURIComponent(date)}`
+    );
+  },
+
+  /**
+   * Create a new exchange/trade
+   */
+  createExchange(request: CreateExchangeRequest): Promise<ExchangeResponse> {
+    return client.post<ExchangeResponse>("/api/exchange", request);
+  },
+};