Add `just backup` and `just restore` recipes

[bBlazewavE]

Summary

Currently just link creates timestamped backups of existing configs, but there's no easy way to list or restore them. Add explicit backup/restore recipes.

Proposed recipes

Recipe	What it does
just backup	Creates a dated snapshot of current configs (~/.config/nvim, ~/.tmux.conf, ~/.zshrc)
just restore	Lists available backups and restores a selected one

Use case

Users experimenting with config changes who want a quick rollback path without reaching for git.

[bBlazewavE]

✅ Done! I've added just backup and just restore recipes for easy config snapshots.

What was added:

1. just backup — Creates a dated snapshot of current configs

   • Backs up ~/.config/nvim, ~/.tmux.conf, and ~/.zshrc
   • Saves to ~/.local/share/shellsmith/backups/TIMESTAMP/

2. just restore — Interactive backup restoration

   • Lists available backups with timestamps
   • Prompts you to select which one to restore
   • Saves a pre-restore snapshot (in case you change your mind)

Use case:
Experiment with config changes, then quickly rollback without touching git:

just backup          # before making changes
# ... edit configs ...
just restore         # oops, revert it all!

Both recipes are idempotent and safe to run repeatedly.