fix(zsh): harden shell bootstrap and XDG path setup

zsh/.zshenv

@@ -1,3 +1,7 @@
-XDG_CONFIG_HOME="${HOME}/.config"
-XDG_BIN_HOME="${HOME}/.local/bin"
-ZDOTDIR="${XDG_CONFIG_HOME}/zsh"
+export XDG_CONFIG_HOME="${XDG_CONFIG_HOME:-$HOME/.config}"
+export XDG_DATA_HOME="${XDG_DATA_HOME:-$HOME/.local/share}"
+export XDG_CACHE_HOME="${XDG_CACHE_HOME:-$HOME/.cache}"
+export XDG_STATE_HOME="${XDG_STATE_HOME:-$HOME/.local/state}"
+
+XDG_BIN_HOME="${XDG_BIN_HOME:-$HOME/.local/bin}"
+ZDOTDIR="$XDG_CONFIG_HOME/zsh"