docs: update README with demo integration details and setup instructions

Author: andrii-harbour

demo/README.md

@@ -1,26 +1,51 @@
 # Demo
 
+This demo shows how to integrate `@superdoc-dev/esign` into a React application. The frontend sends signing and download requests to a proxy server, which securely communicates with the SuperDoc Services API.
+
+## Prerequisites
+
+You'll need a SuperDoc Services API key. [Get your API key here](https://docs.superdoc.dev/api-reference/authentication/register).
+
 ## Setup
 
-1. Build the main package first (from root):
+1. Build the main package (from repo root):
    ```bash
    pnpm build
    ```
 
-2. Install demo dependencies:
+2. Install dependencies:
    ```bash
    cd demo
    pnpm install
+   cd server
+   pnpm install
    ```
 
-3. Create `.env` file with your API key:
+3. Create `.env` file in `demo/server/`:
    ```
-   VITE_SUPERDOC_SERVICES_API_KEY=your_key_here
+   SUPERDOC_SERVICES_API_KEY=your_key_here
    ```
 
-4. Run the demo:
-   ```bash
-   pnpm run dev
+4. Update `demo/vite.config.ts` to proxy to localhost:
+   ```ts
+   proxy: {
+     '/v1': {
+       target: 'http://localhost:3001',
+       changeOrigin: true,
+     },
+   },
    ```
 
+## Running
+
+Start the proxy server:
+```bash
+cd demo/server
+pnpm start
+```
 
+In a separate terminal, start the frontend:
+```bash
+cd demo
+pnpm dev
+```