feat: modernize UI with semantic colors and visual hierarchy improvements

[inureyes]

Summary

This PR modernizes the bssh UI with semantic colors, visual hierarchy improvements, XDG Base Directory specification compliance, and adds Debian packaging support.

Changes

• ✨ Modern UI with semantic colors and Unicode symbols

  • Added color-coded output using owo-colors crate for better visual hierarchy
  • Implemented Unicode symbols (●, ✓, ✗, ⚡, 📂, etc.) for status indicators
  • Created responsive multi-node grid display with terminal size awareness
  • Added OutputFormatter for consistent formatting across all commands

• 📁 XDG Base Directory specification compliance

  • Changed default config path from ~/.bssh/config.yaml to ~/.config/bssh/config.yaml
  • Added XDG_CONFIG_HOME environment variable support
  • Removed legacy path support for cleaner codebase
  • Updated all documentation to reflect new config loading priority

• 📦 Debian packaging support

  • Added complete Debian packaging infrastructure (control files, rules, scripts)
  • Set Rust 1.88 as minimum version requirement
  • Added proper package descriptions and dependencies
  • Included Recommends (openssh-client, openssh-server) and Suggests (ssh-agent, keychain, bash-completion)

• 🏷️ Backend.AI project attribution

  • Added "Developed and maintained as part of the Backend.AI project" to CLI help
  • Updated README.md with Backend.AI attribution
  • Added attribution to manpage AUTHOR section

Test Plan

• Test UI output with single node execution
• Test UI output with multi-node execution
• Verify XDG config path resolution
• Verify environment variable support
• Test color output in different terminal emulators
• Verify help message formatting
• Build Debian package successfully