Add comprehensive export/import capabilities

## Problem
Users need to export data subsets, convert formats, and work with multiple files simultaneously.

## Solution
Implement comprehensive export/import:
- Export filtered/search results
- Export specific subtrees
- Multiple file tabs
- Format conversion (JSON ↔ CSV ↔ YAML)
- Batch operations

## Benefits
- Data extraction and analysis workflows
- Format interoperability
- Multi-file productivity
- Professional data processing features

## Implementation Notes
- Support multiple export formats
- Implement tabbed interface for multiple files
- Add drag-and-drop for file operations
- Include format validation and conversion

## Related Issues
- #8 - JSONPath search functionality (export results moved here)

## Acceptance Criteria
- [ ] Export search results to file
- [ ] Export selected subtrees
- [ ] Tabbed interface for multiple files
- [ ] JSON to CSV conversion for arrays
- [ ] YAML import/export support
- [ ] Drag-and-drop file operations
- [ ] Batch export operations