Implement CLI styling with Charm.sh tools for enhanced user experience

[Copilot]

This PR transforms the md_split CLI tool from basic text output to a modern, visually appealing interface using Charm.sh's ecosystem.

What's Changed

Visual Enhancements

• Colorful Status Messages: Added green ✓ checkmarks for successful file operations, blue 📄 icons for merge operations, and red ✗ symbols for errors
• Real-time Progress Bars: Interactive progress indicators show completion status during file processing with animation and statistics
• Styled Summary Boxes: Professional bordered completion messages that clearly highlight operation results
• Enhanced Error Messages: Red-styled error messages with clear symbols make problems immediately visible

Technical Implementation

• New Style System: Created internal/style package with Lipgloss-based styling functions and a professional color palette
• Progress Tracking: Integrated github.com/schollz/progressbar/v3 for smooth progress visualization
• Modular Design: Clean separation of styling logic from business logic for maintainability
• Backward Compatibility: All original functionality preserved with no breaking changes

Before/After Examples

Before (plain text):

Created: 01-introduction.md
Created: 02-installation.md
Successfully split markdown into 2 files in splits

After (styled output):

ℹ Writing split files...
✓ Created: 01-introduction.md
✓ Created: 02-installation.md
100% |████████████████████| (2/2, 1234 it/s)

╭──────────────────────────────────────────╮
│ Successfully split markdown into 2 files │
╰──────────────────────────────────────────╯

Dependencies Added

• github.com/charmbracelet/lipgloss v1.1.0 - Terminal styling framework
• github.com/schollz/progressbar/v3 v3.18.0 - Progress bar library

All dependencies have been security-scanned and contain no known vulnerabilities.

Testing

• Comprehensive testing across split and merge operations
• Error handling verification with styled output
• File integrity validation (split/merge operations maintain content accuracy)
• No performance impact on core functionality

The CLI now provides a professional, modern user experience while maintaining all original functionality.

---

💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click here to start the survey.

[Copilot]

Pull Request Overview

This PR enhances the md_split CLI tool by integrating Charm.sh's styling ecosystem to provide a modern, visually appealing terminal interface while maintaining backward compatibility.

• Added comprehensive styling system using Lipgloss with color-coded status messages and progress bars
• Implemented real-time progress tracking for file operations using progressbar/v3
• Enhanced error handling with styled output that makes issues immediately visible

Reviewed Changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
internal/style/style.go	New styling package with Lipgloss-based color palette and helper functions
go.mod	Added dependencies for Lipgloss styling and progressbar functionality
cmd/split.go	Enhanced split command with styled output and progress tracking
cmd/root.go	Updated error handling to use styled error messages
cmd/merge.go	Enhanced merge command with styled output and progress tracking

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}