Add PATH persistence and shell command refresh hints to installer

[tingkai-c]

Motivation

• Improve post-install UX by ensuring the installed binary becomes available immediately in common shells.
• Reduce confusion around shell command caches and missing PATH entries after installing localsend-cli.

Description

• Add print_command_refresh_hint to print shell-specific instructions to refresh command lookup for zsh, bash/ksh/ash/dash/sh, fish, and a generic fallback.
• Add persist_path_hint which writes a shell-appropriate PATH update into the user's rc file (~/.zshrc, ~/.bashrc, or ~/.config/fish/config.fish) if the line is not already present and creates the parent directory as needed.
• Call persist_path_hint "$dir" when the chosen install directory is not on PATH, and call print_command_refresh_hint before the final usage hint.
• Ensure the code checks for existing lines before appending and reports additions via info while preserving existing install flow.

Testing

• Ran a syntax check with bash -n install.sh which succeeded.
• Ran static analysis with shellcheck install.sh which reported no critical failures.

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Codex Task