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/_smart_execute.fish

@@ -15,8 +15,8 @@ function _smart_execute --description 'Execute different functions based on the
     # 2. Dispatch based on buffer content
     switch "$cmd"
         case '*='
-            # If it ends in =, run qalc
-            _qalc_eval
+            # If it ends in =, run qalc; fall back to normal execute if qalc is absent
+            _qalc_eval; or commandline -f execute
 
 #       case 'g *'
 #           # EXAMPLE FUTURE EXTENSION