fix(shell): add explicit return statements to functions

Add return 0 as the last statement in ~46 shell functions across
17 files that previously relied on implicit return codes
(SonarCloud rule shelldre:S7682).

scripts/install-apt-packages.sh

@@ -64,12 +64,14 @@ install_packages()
   else
     msgr ok "All packages already installed"
   fi
+  return 0
 }
 
 main()
 {
   install_packages
   msgr yay "apt package installations complete"
+  return 0
 }
 
 main "$@"