docs(frontend): Add API services guide for frontend development (#12132)

frontend/src/api/README.md

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+# API Services Guide
+
+## Overview
+
+Services are the abstraction layer between frontend components and backend APIs. They encapsulate HTTP requests using the shared `openHands` axios instance and provide typed methods for each endpoint.
+
+Each service is a plain object with async methods.
+
+## Structure
+
+Each service lives in its own directory:
+
+```
+src/api/
+├── billing-service/
+│   ├── billing-service.api.ts    # Service methods
+│   └── billing.types.ts          # Types and interfaces
+├── organization-service/
+│   ├── organization-service.api.ts
+│   └── organization.types.ts
+└── open-hands-axios.ts           # Shared axios instance
+```
+
+## Creating a Service
+
+Use an object literal with named export. Use object destructuring for parameters to make calls self-documenting.
+
+```typescript
+// feature-service/feature-service.api.ts
+import { openHands } from "../open-hands-axios";
+import { Feature, CreateFeatureParams } from "./feature.types";
+
+export const featureService = {
+  getFeature: async ({ id }: { id: string }) => {
+    const { data } = await openHands.get<Feature>(`/api/features/${id}`);
+    return data;
+  },
+
+  createFeature: async ({ name, description }: CreateFeatureParams) => {
+    const { data } = await openHands.post<Feature>("/api/features", {
+      name,
+      description,
+    });
+    return data;
+  },
+};
+```
+
+### Types
+
+Define types in a separate file within the same directory:
+
+```typescript
+// feature-service/feature.types.ts
+export interface Feature {
+  id: string;
+  name: string;
+  description: string;
+}
+
+export interface CreateFeatureParams {
+  name: string;
+  description: string;
+}
+```
+
+## Usage
+
+> [!IMPORTANT]
+> **Don't call services directly in components.** Wrap them in TanStack Query hooks.
+>
+> Why? TanStack Query provides:
+> - **Caching** - Avoid redundant network requests
+> - **Deduplication** - Multiple components requesting the same data share one request
+> - **Loading/error states** - Built-in `isLoading`, `isError`, `data` states
+> - **Background refetching** - Data stays fresh automatically
+>
+> Hooks location:
+> - `src/hooks/query/` for data fetching (`useQuery`)
+> - `src/hooks/mutation/` for writes/updates (`useMutation`)
+
+```typescript
+// src/hooks/query/use-feature.ts
+import { useQuery } from "@tanstack/react-query";
+import { featureService } from "#/api/feature-service/feature-service.api";
+
+export const useFeature = (id: string) => {
+  return useQuery({
+    queryKey: ["feature", id],
+    queryFn: () => featureService.getFeature({ id }),
+  });
+};
+```
+
+## Naming Conventions
+
+| Item | Convention | Example |
+|------|------------|---------|
+| Directory | `feature-service/` | `billing-service/` |
+| Service file | `feature-service.api.ts` | `billing-service.api.ts` |
+| Types file | `feature.types.ts` | `billing.types.ts` |
+| Export name | `featureService` | `billingService` |