TUI: Add progress indicators for long operations

[mihaelamj]

Summary

Show visual feedback during long-running operations like artifact scanning and saving.

Current Issues

• Artifact scanning blocks UI with no feedback
• Saving selections appears frozen
• No indication of progress

Design

Spinner animation:

⠹ Scanning library artifacts...

Uses animation frames: ⠋⠙⠹⠸⠼⠴⠦⠧⠇⠏

Tasks

• Create Spinner.swift with animation frames
• Add async progress tracking to artifact scanner
• Show spinner during file I/O operations
• Add status message area in views
• Test spinner doesn't interfere with input

Files

• Sources/TUI/Infrastructure/Spinner.swift (new)
• Sources/TUI/PackageCurator.swift
• Sources/TUI/Views/*.swift