Add sendPhotoFromUrl endpoint for uploading media from URL to groups

[Copilot]

This PR implements a new feature that allows users to send images from URLs directly to Telegram groups through the Cloudflare Worker proxy.

🚀 New Feature: sendPhotoFromUrl

Endpoint: POST /bot{token}/sendPhotoFromUrl

Required Parameters:

• url: Direct URL to the image file
• chat_id: Group or user chat ID to send the image to

Optional Parameters:

• caption: Caption text for the image
• parse_mode: Parse mode for caption (HTML, Markdown, etc.)

📋 Example Usage

fetch('https://your-worker.workers.dev/botYOUR_BOT_TOKEN/sendPhotoFromUrl', {
    method: 'POST',
    headers: {
        'Content-Type': 'application/json',
    },
    body: JSON.stringify({
        url: "https://example.com/image.jpg",
        chat_id: "-1001234567890",
        caption: "<b>Image from URL</b>\nSent using the proxy!",
        parse_mode: "HTML"
    })
})
.then(response => response.json())
.then(data => console.log(data));

🔧 Technical Implementation

• Minimal changes: Added 86 lines of new functionality while preserving all existing proxy behavior
• Comprehensive validation: URL format checking, required parameter validation, and image URL detection
• Error handling: Proper error responses for invalid URLs, missing parameters, and network issues
• CORS support: Maintains existing CORS headers for web applications
• Integration: Uses Telegram's native sendPhoto API with URL support

📚 Documentation Updates

• Updated inline HTML documentation with interactive examples
• Enhanced README.md with Vietnamese usage instructions
• Added code examples and parameter descriptions

🧪 Testing

• URL validation logic tested and verified
• Endpoint routing confirmed working correctly
• Parameter validation implemented and tested
• Code syntax validation passed

The feature is ready for production deployment and maintains full backward compatibility with existing functionality.

Fixes #11.

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