feat(api): generate TypeScript API client from OpenAPI specification

Create type-safe TypeScript API client automatically generated from the
OpenAPI specification. Includes generated schema types and documented client
wrapper for type-safe backend communication.

Ref: T008 (specs/001-modbus-relay-control)

src/api/README.md

@@ -0,0 +1,61 @@
+# API Client
+
+This directory contains the auto-generated TypeScript API client for the STA backend.
+
+## Files
+
+- `schema.ts` - Auto-generated OpenAPI type definitions (do not edit manually)
+- `client.ts` - API client instance with type-safe methods
+
+## Regenerating the Client
+
+To regenerate the TypeScript client after backend API changes:
+
+1. Start the backend server:
+   ```bash
+   cargo run
+   ```
+
+2. Download the OpenAPI spec:
+   ```bash
+   curl http://localhost:3100/specs > openapi.yaml
+   ```
+
+3. Generate TypeScript types:
+   ```bash
+   pnpm exec openapi-typescript openapi.yaml -o src/api/schema.ts
+   ```
+
+## Usage Example
+
+```typescript
+import { apiClient } from '@/api/client';
+
+// GET request
+const { data, error } = await apiClient.GET('/api/health');
+if (error) {
+  console.error('Health check failed:', error);
+} else {
+  console.log('Server is healthy');
+}
+
+// GET request with response data
+const { data: meta, error: metaError } = await apiClient.GET('/api/meta');
+if (metaError) {
+  console.error('Failed to get metadata:', metaError);
+} else {
+  console.log('App name:', meta.name);
+  console.log('App version:', meta.version);
+}
+```
+
+## Configuration
+
+The API base URL can be configured via the `VITE_API_BASE_URL` environment variable.
+Create a `.env` file in the project root:
+
+```env
+VITE_API_BASE_URL=http://localhost:3100
+```
+
+For production builds, set the environment variable to point to your deployed backend.