fix(install): make setup failures explicit and atomic

Installer pipelines masked dependency and download failures, causing
false success messages and unsafe config replacement.

Atomic updates and current upstream installers keep failures visible.

zshrc/fnm.zsh

@@ -1,8 +1,6 @@
-# fnm (Fast Node Manager) configuration
-# https://github.com/Schniz/fnm
-
-FNM_PATH="$HOME/.local/share/fnm"
-if [ -d "$FNM_PATH" ]; then
-  export PATH="$HOME/.local/share/fnm:$PATH"
-  eval "$(fnm env --use-on-cd --shell zsh)"
+# fnm (Fast Node Manager)
+FNM_PATH="${XDG_DATA_HOME:-$HOME/.local/share}/fnm"
+[[ -d "$FNM_PATH" ]] && export PATH="$FNM_PATH:$PATH"
+if command -v fnm &> /dev/null; then
+    eval "$(fnm env --use-on-cd --shell zsh)"
 fi