fix(scripts): add missing exit after error handlers

Critical bugs where error paths print a message but don't stop execution:
- install-fonts.sh: cd failure now exits properly
- install-ntfy.sh: unsupported OS case now exits with error
- install-git-crypt.sh: git clone and cd failures now exit properly

scripts/install-ntfy.sh

@@ -23,6 +23,7 @@ case $(dfm check arch) in
     ;;
   *)
     msgr err "Unsupported OS"
+    exit 1
     ;;
 esac