[Feature] Add automatic code formatting with Prettier

[kom50]

Summary

One-click code formatting using Prettier.

Requirements

• Format button in header
• Keyboard shortcut: Shift + Alt + F
• Support JS, TS, HTML, CSS, JSON, Markdown
• Show error toast if formatting fails
• Preserve cursor position after format

Technical Notes

<script src="https://unpkg.com/prettier@3.0.0/standalone.js"></script>
<script src="https://unpkg.com/prettier@3.0.0/plugins/babel.js"></script>

Acceptance Criteria

• Code is formatted on button click
• Formatting works for supported languages
• Errors are handled gracefully