Implement folder merge functionality

[ggfevans]

Fixes #5

Summary

Implements smart folder merging when archiving a folder that already exists in the archive destination.

Changes

• Prompts user to merge when archiving folder with existing destination
• Compares file contents byte-by-byte before replacing
• Skips identical files to preserve existing versions
• Replaces only files with different content
• Tracks detailed merge statistics (added, replaced, skipped, failed)
• Handles individual file errors gracefully without stopping merge
• Only deletes source folder when completely empty

Implementation Details

• Iterates over copy of children array to prevent modification-during-iteration issues
• Uses app.vault.read() for content comparison
• Reports detailed statistics in success message
• Lists failed files if any errors occur
• Prevents ENOTEMPTY errors with empty-folder check before deletion

Testing

Tested with mixed-content folders containing:

• Identical files (skipped correctly)
• Modified files (replaced correctly)
• New files (added correctly)

All merge operations completed successfully with proper statistics reporting.