zsh: escape double-quoted values

The zsh value renderer emits strings in double quotes so session variables can still expand references such as $HOME. Embedded quotes, backslashes, and backticks were not escaped, which could corrupt generated zsh code or trigger command substitution.

Escape those double-quote-sensitive characters while preserving dollar expansion for existing session-variable semantics.

modules/lib/zsh.nix

@@ -7,7 +7,7 @@ rec {
     if builtins.isBool v then
       if v then "true" else "false"
     else if builtins.isString v then
-      ''"${v}"''
+      ''"${lib.escape [ "\\" "\"" "`" ] v}"''
     else if builtins.isList v then
       let
         shell = import ./shell.nix { inherit lib; };