fix: prevent path traversal in extract_har.py (fixes #320)

[KooshaPari]

Fixes #320

Changes

• Added is_safe_path() helper to verify output paths resolve within the output directory
• Added path traversal sanitization in extract_path_from_url() to strip ../ components
• Added security check before writing each file to prevent directory traversal attacks
• Skips files with unsafe paths and logs a warning instead of crashing

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