fix: add encoding=utf-8 to CLI file operations

[nightcityblade]

Summary

On Windows, the default file encoding (e.g. charmap / cp1252) cannot encode certain Unicode characters found in crawled content, causing 'charmap' codec can't encode character errors when writing output files via the CLI.

Fixes #1762

List of files changed and why

crawl4ai/cli.py — Added encoding="utf-8" to all 6 open() calls (config read/write + 4 output file writes)

How Has This Been Tested?

• Code review: all file I/O paths in CLI now explicitly use UTF-8
• The fix matches Python best practice for cross-platform Unicode file handling

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added/updated unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes