feat: add File Converter template (#12)

[lout33]

Summary

Adds a self-hosted File Converter template for Codesphere workspaces.

Features

• Image conversion between JPG, PNG, WebP, GIF, AVIF formats
• Batch processing (up to 10 files at once)
• Token-based authentication
• Privacy-first: all processing happens locally
• Express.js backend with sharp for image processing
• Clean web UI with drag-and-drop upload

Payout

• 1000 Codesphere credits
• 50$ cash (per issue File converter #12)

Files

• file-converter/server.js - Express server with conversion API
• file-converter/src/utils/imageConverter.js - Sharp-based image conversion
• file-converter/src/middleware/auth.js - Token authentication
• file-converter/ci.yml - Codesphere CI pipeline
• file-converter/metadata.json - template metadata
• file-converter/README.md - setup instructions
• file-converter/start.sh - startup script

Deployment

1. Fork this template
2. Create Codesphere workspace from fork
3. Run prepare stage to install dependencies
4. Run run stage to start server
5. Open deployment to use the tool

Closes #12