Improve Linux prompt setup, themes, and uninstall safety

[joematrix77]

Summary

• install Starship and JetBrainsMono Nerd Font on Linux
• configure Ptyxis and GNOME Terminal to use the Nerd Font while backing up prior font settings
• restore the newest timestamped .bashrc backup during uninstall
• add uninstall.sh --keep-deps to retain installed software and fonts
• add the starship-theme palette switcher and palette preview

Scope cleanup

This branch was rebuilt from current main. It no longer reverts .bashrc, the Actions workflow, Fastfetch configuration, current installer portability, or macOS support. Fork branding, fork URLs, the clone-layout replacement, the .bashrc_personal loader, root refusal, and the unrelated prompt screenshot were removed.

Safety

• the Starship installer is downloaded to a temporary file before execution instead of being piped directly into a shell
• existing terminal font settings are saved and restored during uninstall
• the theme switcher generates a user-owned config from the pristine tracked theme without modifying the repository copy

Validation

• sh -n setup.sh uninstall.sh
• bash -n starship-theme
• shellcheck setup.sh uninstall.sh starship-theme
• shfmt -d setup.sh uninstall.sh starship-theme
• all 12 palettes applied against the current starship.toml
• timestamped .bashrc restoration and --keep-deps behavior tested in an isolated home
• Ptyxis and GNOME Terminal backup/restore paths tested with mocked gsettings
• current Nerd Fonts archive contents and Starship installer flags verified