validate downloaded zshrc with zsh -n before replacing

Adds syntax validation via zsh -n to both update_zshrc() and the
auto-update on shell start. Prevents error pages or corrupted content
from overwriting ~/.zshrc and breaking the shell.

zsh/zshrc

@@ -17,7 +17,7 @@ export ZSHRC_BOOTSTRAP="$ZSH_CONFIG/.bootstrapped"
 update_zshrc(){
   local tmp=/tmp/.zshrc.$$
   curl -sSL --connect-timeout 5 "$ZSHRC_URL" -o "$tmp" 2>/dev/null || return
-  if ! cmp -s "$tmp" "$HOME/.zshrc" 2>/dev/null; then
+  if zsh -n "$tmp" 2>/dev/null && ! cmp -s "$tmp" "$HOME/.zshrc" 2>/dev/null; then
     echo "Updating zshrc..."
     mv "$tmp" "$HOME/.zshrc" && echo "Updated. Reloading..." && exec zsh
   fi
@@ -127,7 +127,7 @@ alias yeet='yay -Rcs'
 if [[ -o interactive ]]; then
   tmp=/tmp/.zshrc.$$
   curl -sSL --connect-timeout 3 "$ZSHRC_URL" -o "$tmp" 2>/dev/null
-  if [[ -f "$tmp" ]] && ! cmp -s "$tmp" "$HOME/.zshrc" 2>/dev/null; then
+  if [[ -f "$tmp" ]] && zsh -n "$tmp" 2>/dev/null && ! cmp -s "$tmp" "$HOME/.zshrc" 2>/dev/null; then
     echo "Updating zshrc..."
     mv "$tmp" "$HOME/.zshrc" && exec zsh
   fi