[Feature] Download code as file

[kom50]

Summary

Allow users to download the current editor content as a file with appropriate extension.

Requirements

• Add "Download" button in header
• Auto-detect file extension from language mode
• Generate filename: collabpad-{roomId}.{ext}
• Support custom filename input (optional)
• Trigger browser download

File Extension Mapping

Language	Extension
JavaScript	.js
TypeScript	.ts
Python	.py
HTML	.html
CSS	.css
Markdown	.md
JSON	.json
SQL	.sql

Technical Notes

function downloadFile(content, filename) {
  const blob = new Blob([content], { type: 'text/plain' });
  const url = URL.createObjectURL(blob);
  const a = document.createElement('a');
  a.href = url;
  a.download = filename;
  a.click();
  URL.revokeObjectURL(url);
}

Acceptance Criteria

• Clicking download saves file to user's device
• File extension matches current language
• File content matches editor content exactly