feat(keybinds): move window focus to SUPER+arrows

Swap the vim-style SUPER+H/J/K/L focus binds for SUPER+arrow keys
(movefocus l/r/u/d). SUPER+arrows were unbound and there are no vim
move-window binds, so no collision. prettyKeys renders the arrow keysyms
as glyphs so the SUPER+? cheatsheet shows "SUPER + <-" not "SUPER + left".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/home/rofi.nix

@@ -126,7 +126,9 @@ let
   prettyKeys = b:
     let m = lib.replaceStrings [ "$mod" ] [ "SUPER" ] b.mods;
         prefix = if m == "" then "" else lib.concatStringsSep " + " (lib.splitString " " m) + " + ";
-        key = lib.replaceStrings [ "question" "slash" ] [ "?" "/" ] b.key;
+        key = lib.replaceStrings
+          [ "question" "slash" "left" "right" "up" "down" ]
+          [ "?" "/" "←" "→" "↑" "↓" ] b.key;
         # ? already implies Shift — the bind needs SHIFT in its modmask
         # (keybinds.nix), but showing it would read double, so collapse
         # "SHIFT + ?" back to the documented "?".