Add ajax-demo example and update @formspree/ajax API

- Replace formEndpoint with simpler formId + optional origin config
- Add Vite-based demo app with contact form UI
- Include loading states, error handling, and form validation feedback
- Export appendExtraData from @formspree/core for extra form data

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: lpssformspree

examples/ajax-demo/.env.local.template

@@ -0,0 +1,3 @@
+# Formspree Configuration
+# Copy this file to .env.local and add your form ID
+VITE_FORMSPREE_FORM_ID=YOUR_FORM_ID

examples/ajax-demo/.eslintrc.yml

@@ -0,0 +1,5 @@
+root: true
+extends:
+  - '../../.eslintrc.yml'
+parserOptions:
+  project: './tsconfig.json'

examples/ajax-demo/README.md

@@ -0,0 +1,125 @@
+# Formspree AJAX Demo
+
+A pure JavaScript/TypeScript example demonstrating how to use `@formspree/ajax` for form submissions without any frontend framework.
+
+## Features
+
+- Pure HTML/CSS/TypeScript implementation
+- No React, Vue, or other framework dependencies
+- Beautiful, responsive contact form UI
+- Loading states and error handling
+- Form validation feedback
+- Success/error messages
+- Built with Vite for fast development
+
+## Setup
+
+1. Install dependencies:
+
+   ```bash
+   yarn install
+   ```
+
+2. Get your Formspree form ID:
+
+   - Go to [formspree.io](https://formspree.io)
+   - Create a new form or use an existing one
+   - Copy your form ID (e.g., `xyzabc123`)
+
+3. Create a `.env.local` file from the template:
+
+   ```bash
+   cp .env.local.template .env.local
+   ```
+
+4. Update `VITE_FORMSPREE_FORM_ID` in `.env.local` with your form ID.
+
+## Development
+
+Run the development server:
+
+```bash
+yarn dev
+```
+
+The demo will be available at `http://localhost:5173`
+
+## Build
+
+Build for production:
+
+```bash
+yarn build
+```
+
+The output will be in the `dist` directory.
+
+## Usage
+
+### Basic Form Initialization
+
+```typescript
+import { initForm } from '@formspree/ajax';
+
+initForm({
+  formElement: '#contact-form',
+  formId: 'your-form-id',
+  onSuccess: ({ form }) => {
+    console.log('Success!');
+    form.reset();
+  },
+  onError: (_context, error) => {
+    const messages = error.getFormErrors().map((e) => e.message);
+    console.error('Validation errors:', messages);
+  },
+  onFailure: (_context, error) => {
+    console.error('Unexpected error:', error);
+  },
+});
+```
+
+### With Extra Data
+
+```typescript
+initForm({
+  formElement: '#contact-form',
+  formId: 'your-form-id',
+  data: {
+    source: 'website',
+    timestamp: new Date().toISOString(),
+  },
+  onSuccess: ({ form }) => {
+    form.reset();
+  },
+});
+```
+
+### Custom Origin (e.g., staging)
+
+```typescript
+initForm({
+  formElement: '#contact-form',
+  formId: 'your-form-id',
+  origin: 'https://staging.formspree.io',
+  debug: true,
+});
+```
+
+## File Structure
+
+```
+ajax-demo/
+├── public/
+│   ├── index.html      # HTML form with styling
+│   └── main.ts         # TypeScript implementation
+├── .env.local.template # Environment variables template
+├── package.json
+├── tsconfig.json
+├── vite.config.ts
+└── README.md
+```
+
+## Learn More
+
+- [Formspree Documentation](https://help.formspree.io/)
+- [@formspree/ajax Package](../../packages/formspree-ajax)