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/key_bindings.fish

@@ -52,7 +52,7 @@ function fish_user_key_bindings
     bind ctrl-g __insert_previous_path_head
     bind ctrl-f __interactive_history_sub
     bind ctrl-alt-u _replace_command_token
-    bind ctrl-alt-= _qalc_eval
+    type -q qalc && bind ctrl-alt-= _qalc_eval
     bind ctrl-enter _smart_execute
 
     # Set bindings for all Vi modes:
@@ -61,7 +61,7 @@ function fish_user_key_bindings
         bind --mode $mode ctrl-g __insert_previous_path_head
         bind --mode $mode ctrl-f __interactive_history_sub
         bind --mode $mode ctrl-alt-u _replace_command_token
-        bind --mode $mode ctrl-alt-= _qalc_eval
+        type -q qalc && bind --mode $mode ctrl-alt-= _qalc_eval
         bind --mode $mode ctrl-enter _smart_execute
     end
 end