refactor(install): improve modular architecture and error handling

- Remove redundant installation checks in arduino.sh, git.sh
- Add bash shell enforcement in install.sh to prevent compatibility issues
- Streamline Arduino CLI installation logic for both Arch and other systems
- Improve Git and Lazygit installation messaging consistency
- Update install usage instructions for better clarity
- Enhance shell compatibility by enforcing bash execution

lib/starship.sh

@@ -3,15 +3,14 @@
 
 step "3/9 — Installing Starship"
 
-if ! command -v starship &>/dev/null; then
-  if $IS_ARCH; then
-    $PKG_INSTALL starship
-  else
-    curl -ksS https://starship.rs/install.sh | sh -s -- -y
-  fi
+if $IS_ARCH; then
+  $PKG_INSTALL starship
+else
+  curl -ksS https://starship.rs/install.sh | sh -s -- -y
 fi
-info "Starship installed"
+info "Starship installed/updated"
 
 mkdir -p ~/.config
+backup_config ~/.config/starship.toml
 write_starship_config ~/.config/starship.toml
 info "Starship configured with $THEME_DISPLAY_NAME"