Add more helpful error messages for common CLI mistakes

Make error messages more user-friendly when users make common mistakes.

Tasks:

• Add specific error when file doesn't exist
• Add hint when Ollama is not running (for PDF→MD)
• Add suggestion when wrong file type is provided
• Improve error formatting with colors (using chalk)

Files to modify:

• src/cli.ts
• src/converters/pdf-to-markdown.ts

Good First Issue: Simple TypeScript changes with clear requirements!