Feature: Add --json output mode and scan report export

## Summary
Two related output improvements:

1. **--json flag** for all commands - Enable pipeline-friendly JSON output for integration with other tools
2. **Scan report export** - Export scan results to JSON/HTML format

## Use Cases
- `jwt-hack decode --json <token> | jq .payload`
- `jwt-hack scan --report report.json <token>`
- CI/CD integration with structured output