Add gum TUI with auto-install, preflight checks, and beautiful interactive menu

[Copilot]

Replaces the plain read-based menu with a charmbracelet/gum TUI while keeping all distro ASCII art and installation logic intact. Falls back to the original terminal UI if gum cannot be installed.

Auto-install gum

• ensure_gum() installs gum via pacman -S gum (Arch extra repo) with AUR helper fallback (yay/paru)
• Sets GUM_AVAILABLE flag; all gum paths degrade gracefully when unavailable

Per-distro accent colours

• GUM_DISTRO_COLOR (256-colour index) added to detect_system() alongside existing DISTRO_STYLE
• Used consistently across gum borders and styled text throughout the TUI

Preflight checks

• run_preflight_checks() runs before the menu and shows live ✓/✗/⚠ status for: root, internet (gum spin spinner), disk space (< 10 GB warns), and pacman availability
• Extracted _has_internet() as a shared silent helper used by both check_internet() (fallback mode) and run_preflight_checks() — eliminates the duplicated scoring logic

Interactive multi-select menu

• gum choose --no-limit — all 20 options with emoji + [Category] prefix; ↑/↓ navigate, x/space toggle, Enter confirm
• Distro ASCII art + system info panel (distro-accent bordered) shown above the menu
• mapfile used for array deduplication to avoid word-splitting edge cases

Confirmation & install flow

• confirm_selections_gum() renders selected components in a green rounded-border checklist, confirmed via gum confirm with Install / Cancel
• gum_section() helper provides styled section headers during installation; success/failure banners updated to use gum style
• Post-install prompt offers Return to Menu / Exit via gum confirm
• Ctrl+C (exit 130) and empty selection (exit 1) handled distinctly in main()