fix(keybinds): guard bindings on required binaries and fix fzf bg-transform

- conf.d/fzf.fish: skip fzf_configure_bindings if fzf is not in PATH
- key_bindings.fish: only bind Ctrl+Alt+= when qalc is installed
- _qalc_eval: return 1 early if qalc is absent so callers can react
- _smart_execute: fall back to normal execute when _qalc_eval returns 1
- integrations/fzf.fish: replace bg-transform with transform (available
  since fzf 0.53; bg-transform requires a newer version and caused
  "unknown action" errors on fzf 0.60 devel)

conf.d/fzf.fish

@@ -9,7 +9,8 @@ end
 # This variable is global so that it can be referenced by fzf_configure_bindings and in tests
 set --global _fzf_search_vars_command '_fzf_search_variables (set --show | psub) (set --names | psub)'
 
-# Install the default bindings, which are mnemonic and minimally conflict with fish's preset bindings
+# Install the default bindings only if fzf is available
+type -q fzf || exit
 fzf_configure_bindings
 
 # Doesn't erase autoloaded _fzf_* functions because they are not easily accessible once key bindings are erased