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)

functions/_qalc_eval.fish

@@ -3,6 +3,8 @@
 
 # Returns the result of a qalc calculation
 function _qalc_eval
+    type -q qalc || return 1
+
     # Get the current command line buffer
     set -l cmd (commandline)