fix(rofi): Display menu Back returns to System on single-monitor

The mode picker's Back did `exec "$0" display`. On a single-monitor
machine with no display profiles the output chooser is skipped, so
re-running `display` dropped straight back into the same mode picker — an
apparent no-op. Track modeBack (display when the chooser was shown, else
system) and dispatch Back to it, so single-monitor Back leaves Display.

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

modules/home/rofi.nix

@@ -335,6 +335,11 @@ ${themeRows}
 
         # Choose the output (skip the chooser when only one is connected and
         # there are no profiles — the Profiles row needs the chooser to live in).
+        # Where the mode picker's Back returns to: the output chooser when it
+        # was shown (a real previous level), else straight to System — on a
+        # single-monitor laptop the chooser is skipped, so returning to
+        # `display` would just re-open this same mode picker (item 40).
+        modeBack=system
         profs=; command -v nomarchy-display-profile >/dev/null 2>&1 && profs=1
         if [ -n "$profs" ] || [ "$(printf '%s' "$mons" | jq -r '.[].name' | grep -c .)" -gt 1 ]; then
           name=$( {
@@ -345,6 +350,7 @@ ${themeRows}
           [ "$name" = "$BACK" ] && exec "$0" system
           [ "$name" = "Profiles ›" ] && exec "$0" display-profile
           name=''${name%%  ·*}                       # strip the "  ·  WxH@R" hint
+          modeBack=display
         else
           name=$(printf '%s' "$mons" | jq -r '.[0].name')
         fi
@@ -361,7 +367,7 @@ ${themeRows}
             | sed 's/Hz$//' | awk -F@ '{ printf "%s@%d\n", $1, $2 + 0.5 }' | sort -rV -u
           printf '%s\n' "$BACK"
         } | rofi -dmenu -p "$name (now $cur)" ) || exit 0
-        [ "$mode" = "$BACK" ] && exec "$0" display
+        [ "$mode" = "$BACK" ] && exec "$0" "$modeBack"
         [ -n "$mode" ] || exit 0
 
         # Apply live (keep the output's current position + scale), then persist.