feat: add hidden PNG graph generator tool

[Paulopk0]

📋 Description

Introduces an automated engine to convert HTML/SVG graphs into static PNG images using html2canvas. This replaces heavy dynamic iframes with lightweight images, significantly improving client-side rendering performance.

🛠️ Changes

1. 📸 HTML to PNG Conversion Engine

• Built an off-screen DOM sandbox (100x50, overflow hidden) to render graphs in isolation. Integrated html2canvas to capture the rendered HTML/SVG elements, generate Base64 PNG images, and send them to save_graph.php for server-side saving. Implemented fallback handling for missing graphs.

2. 🔄 Execution Loop & Rate Limiting

• Added an automated loop to process tickers sequentially, featuring a 1-second delay between conversions and a 5-second pause between full cycles to optimize server resource usage during batch processing.

3. 🧱 Code Structure

• Structured the converter using clean async functions (fetchGraphHtml, prepareSandbox, captureGraph, saveToServer) to maintain clear separation of responsibilities within the conversion pipeline.