feat(cli): add styled output with theme-aware colors

[echobt]

Summary

This PR adds a new styled_output module that provides human-friendly, colorful CLI messages that automatically adapt to the terminal theme and respect the NO_COLOR environment variable.

Changes

New Module: styled_output.rs

• Provides print_success, print_error, print_warning, print_info, and print_dim functions
• Messages use themed colors that adapt to light/dark terminal backgrounds:
  • Success: green (✓ icon)
  • Error: red (✗ icon)
  • Warning: amber (⚠ icon)
  • Info: blue (ℹ icon)
  • Dim: gray (· icon)
• Automatically detects terminal theme via:
  • COLORFGBG environment variable
  • ITERM_PROFILE environment variable
• Respects NO_COLOR environment variable
• Includes format functions that return strings without printing

Updated Files

• login.rs: Login, logout, and status messages now use styled output
• main.rs: Session resume, server status, and mDNS messages use styled output
• import_cmd.rs: Import success/warning messages use styled output
• run_cmd.rs: Session sharing and clipboard messages use styled output
• uninstall_cmd.rs: Backup and cleanup messages use styled output
• github_cmd.rs: GitHub status messages use styled output

Before vs After

Before:

Successfully logged out
Warning: Failed to start mDNS advertising: ...

After:

✓ Logged out successfully. Credentials have been removed.
⚠ Failed to start mDNS advertising: ...

The messages are now more human-friendly with clear visual indicators and consistent styling across the CLI.