fix(installer): install carapace from static release on Arch

[lautarorisso]

Problem

The installer fails on Arch-based distros (Arch, CachyOS, etc.) during the shell step:
sudo pacman -S --needed --noconfirm fish carapace zoxide atuin starship aborts with error: target not found: carapace because carapace is not in the official Arch/CachyOS repositories (AUR-only). Without Homebrew installed the brew fallback is skipped, so the whole installation stops.

Fix

• Removed carapace from the Arch pacman package list in stepInstallShell (fish/zsh/nushell).
• Added installCarapaceBinary which downloads the static release binary (v1.7.3) from the official carapace-bin GitHub releases to ~/.local/bin/carapace, verifying the SHA256 checksum — same pattern as installHerdrBinary.
• The binary is called from stepInstallShell on Arch when carapace is not already on PATH.
• Documented carapace availability on Arch in docs/manual-installation.md.

Testing

TestInstallPlatformPackagesArchFallsBackToBrewWhenNativeFails updated for the new pacman command and passing:
go test ./internal/tui/ -run TestInstallPlatformPackages ✓

Verified end-to-end on CachyOS: carapace 1.7.3 installed to ~/.local/bin/carapace and loaded correctly from zsh (source <(carapace _carapace)).